xen/efi: make Xen PV EFI clock optional

[ehem_freebsd_m5p.com]

Summary

The present implementation is only for x86. Other architectures need adjustments for querying presence of EFI.

Xen's EFI support is also quite troublesome on non-x86. This is being
slowly remedied, but until in better shape the EFI clock functionality
should be disabled.

[ehem_freebsd_m5p.com]

I was taking some glances at making this cross-architecture, but the simpler short-term approach is to make this !arm64. Plus for that matter, Xen's EFI/ACPI support on arm64 isn't yet in particularly good shape.

[ehem_freebsd_m5p.com]

Another alternative would be to have this depend on xentimer, but I've got no idea whether that would be correct (in fact I was under the impression they're quite distinct, so doing that would be an ugly hack).

[ehem_freebsd_m5p.com]

Ideal solution would be to fix D28619. This though requires merging "bootmethod" from x86 with "arm64_bus_method" from ARM64.

Thing is bootmethod is a string, arm64_bus_method is an enum. The enum seems to make more sense. I'm though curious why the somewhat similar vm_guest variable is declared as an int, even though its values are declared as enums.

Testing confirmed making pvefi.c depend on xentimer works, but I'm unsure whether that is correct.

[ehem_freebsd_m5p.com]

I'm marking D31065 as abandoned as D31955 fixes the root cause. D31065 could be resurrected if appropriate, but somehow I don't expect that to happen.

[ehem_freebsd_m5p.com]

Bringing this back in as D31955 has been stuck. Longer term D31955 is better, just this has a better shot at making 14.0.