govc: add -b flag to volume.ls

[dougm]

cns: add json tags

PR #3007 added camelCase json tags for generated types
PR #3247 added camelCase json tags for govc commands

This change does the same for cns/types.

Testing of the -b flag has been done manually for now, as we still don't have a govc volume.create command.
A volume's backing file can already being seen using -json:

% govc volume.ls -json a24b36ff-c407-46ef-b068-a9620145a0cb | jq -r .volume[].backingObjectDetails.backingDiskPath
[sharedVmfs-0] fcd/861ed49f57e549c3a5ea25780c13fdf7.vmdk

With the -b flag, backing is included in the friendly output and also uses QueryVolumeInfo to verify the backing file exists, for example:

% govc volume.ls -b
0b9d71d2-2236-4d67-96e7-89a46292e430	pvc-c3b456c7-01d8-4576-b3eb-6c1b0833fcea	[sharedVmfs-0] fcd/f10e825975bb4b079cba05ca8da86116.vmdk
a24b36ff-c407-46ef-b068-a9620145a0cb	pvc-6584bf4c-fe95-4451-bb45-6d4a5b03b88a	Failed to retrieve the vstorage object for fcd id a24b36ff-c407-46ef-b068-a9620145a0cb
e777a2db-84b9-4a7b-a967-fde885b61592	pvc-01ba5707-6333-49e9-b8e3-abe3654ff2e6	[sharedVmfs-0] fcd/4a601cd0caef43a39fe886e2794dbab9.vmdk

The QueryVolumeInfo task result can also be output as Go code, example with fault:

// govc volume.ls -b -dump a24b36ff-c407-46ef-b068-a9620145a0cb
[]types.BaseCnsVolumeOperationResult{
    &types.CnsQueryVolumeInfoResult{
        CnsVolumeOperationResult: types.CnsVolumeOperationResult{
            VolumeId: types.CnsVolumeId{},
            Fault:    &types.LocalizedMethodFault{
                Fault: types.CnsFault{
                    BaseMethodFault: nil,
                    Reason:          "Failed to retrieve the vstorage object for fcd id a24b36ff-c407-46ef-b068-a9620145a0cb",
                },
                LocalizedMessage: "CnsFault error: Failed to retrieve the vstorage object for fcd id a24b36ff-c407-46ef-b068-a9620145a0cb",
            },
        },
        VolumeInfo: nil,
    },
}

Example when found:

// govc volume.ls -b -dump 0b9d71d2-2236-4d67-96e7-89a46292e430
[]types.BaseCnsVolumeOperationResult{
    &types.CnsQueryVolumeInfoResult{
        CnsVolumeOperationResult: types.CnsVolumeOperationResult{},
        VolumeInfo:               &types.CnsBlockVolumeInfo{
            CnsVolumeInfo:  types.CnsVolumeInfo{},
            VStorageObject: types.VStorageObject{
                Config: types.VStorageObjectConfigInfo{
                    BaseConfigInfo: types.BaseConfigInfo{
                        Id: types.ID{
                            Id: "0b9d71d2-2236-4d67-96e7-89a46292e430",
                        },
                        Name:                        "pvc-c3b456c7-01d8-4576-b3eb-6c1b0833fcea",
                        CreateTime:                  time.Now(),
                        KeepAfterDeleteVm:           types.NewBool(true),
                        RelocationDisabled:          types.NewBool(false),
                        NativeSnapshotSupported:     types.NewBool(false),
                        ChangedBlockTrackingEnabled: types.NewBool(false),
                        Backing:                     &types.BaseConfigInfoDiskFileBackingInfo{
                            BaseConfigInfoFileBackingInfo: types.BaseConfigInfoFileBackingInfo{
                                BaseConfigInfoBackingInfo: types.BaseConfigInfoBackingInfo{
                                    Datastore: types.ManagedObjectReference{Type:"Datastore", Value:"datastore-42", ServerGUID:""},
                                },
                                FilePath:        "[sharedVmfs-0] fcd/f10e825975bb4b079cba05ca8da86116.vmdk",
                                BackingObjectId: "",
                                Parent:          nil,
                                DeltaSizeInMB:   0,
                                KeyId:           (*types.CryptoKeyId)(nil),
                            },
                            ProvisioningType: "thin",
                        },
                        Metadata: nil,
                        Vclock:   (*types.VslmVClockInfo)(nil),
                        Iofilter: nil,
                    },
                    DescriptorVersion: 5,
                    CapacityInMB:      100,
                    ConsumptionType:   []string{"disk"},
                    ConsumerId:        []types.ID{
                        {
                            Id: "6bf965f5-0348-4687-beca-8c89a0624168",
                        },
                    },
                },
            },
        },
    },
}