xen-hypervisor: provide a debug=y build of Xen

[royger]

The problem you're addressing (if any)

When attempting to debug a Xen issue it's common practice to ask the user to switch to a debug build of Xen (a Xen hypervisor built with CONFIG_DEBUG=y). Building such debug enabled hypervisor can be difficult for users, so it would be best to provide one in QubesOS.

The solution you'd like

Users should be able to switch to a debug build of Xen without having to resort to build it themselves. FreeBSD for example builds the hypervisor twice to create a debug=n and debug=y versions of Xen, and installs the later as xen-debug:

https://cgit.freebsd.org/ports/tree/emulators/xen-kernel/Makefile

The value to a user, and who that user might be

Make it easier to switch to debug build of Xen, and as a result provide more helpful information for debugging hypervisor issues.

Completion criteria checklist

[marmarek]

Almost duplicate of #5989

Anyway, there is a PR implementing this already: QubesOS/qubes-vmm-xen#170. But it's blocked on bootloader ordering: QubesOS/qubes-vmm-xen#170 (comment)