Fix VirtualizationWithContext() race in linux

[rmfitzpatrick]

The current common linux VirtualizationWithContext() function* isn't thread safe. These changes introduce a cache singleton to prevent data races:

With only test update:

root@1a49945dd9fa:/go/src/github.com/shirou/gopsutil/host# go test -race -run=TestVirtualization
==================
WARNING: DATA RACE
Write at 0x0000007d8548 by goroutine 8:
  github.com/shirou/gopsutil/internal/common.VirtualizationWithContext()
      /go/src/github.com/shirou/gopsutil/internal/common/common_linux.go:243 +0x933
  github.com/shirou/gopsutil/host.VirtualizationWithContext()
      /go/src/github.com/shirou/gopsutil/host/host_linux.go:365 +0x5a
  github.com/shirou/gopsutil/host.Virtualization()
      /go/src/github.com/shirou/gopsutil/host/host.go:141 +0x97
  github.com/shirou/gopsutil/host.TestVirtualization.func1()
      /go/src/github.com/shirou/gopsutil/host/host_test.go:153 +0x44

Previous read at 0x0000007d8548 by goroutine 14:
  github.com/shirou/gopsutil/internal/common.VirtualizationWithContext()
      /go/src/github.com/shirou/gopsutil/internal/common/common_linux.go:117 +0x52
  github.com/shirou/gopsutil/host.VirtualizationWithContext()
      /go/src/github.com/shirou/gopsutil/host/host_linux.go:365 +0x5a
  github.com/shirou/gopsutil/host.Virtualization()
      /go/src/github.com/shirou/gopsutil/host/host.go:141 +0x97
  github.com/shirou/gopsutil/host.TestVirtualization.func1()
      /go/src/github.com/shirou/gopsutil/host/host_test.go:153 +0x44

Goroutine 8 (running) created at:
  github.com/shirou/gopsutil/host.TestVirtualization()
      /go/src/github.com/shirou/gopsutil/host/host_test.go:152 +0xbe
  testing.tRunner()
      /usr/local/go/src/testing/testing.go:1054 +0x1eb

Goroutine 14 (running) created at:
  github.com/shirou/gopsutil/host.TestVirtualization()
      /go/src/github.com/shirou/gopsutil/host/host_test.go:152 +0xbe
  testing.tRunner()
      /usr/local/go/src/testing/testing.go:1054 +0x1eb
==================

With complete diff:

root@1a49945dd9fa:/go/src/github.com/shirou/gopsutil/host# go test -race -run=TestVirtualization
PASS
ok  	github.com/shirou/gopsutil/host	0.044s

[shirou]

I can reproduce race problem with go test -race -run=TestVirtualization on 1.15.2. Thank you!