xen: Implement EFI reset_system callback

When rebooting DOM0 with ACPI on ARM64, the kernel is crashing with the stack
trace [1].

This is happening because when EFI runtimes are enabled, the reset code
(see machine_restart) will first try to use EFI restart method.

However, the EFI restart code is expecting the reset_system callback to
be always set. This is not the case for Xen and will lead to crash.

The EFI restart helper is used in multiple places and some of them don't
not have fallback (see machine_power_off). So implement reset_system
callback as a call to xen_reboot when using EFI Xen.

[   36.999270] reboot: Restarting system
[   37.002921] Internal error: Attempting to execute userspace memory: 86000004 [#1] PREEMPT SMP
[   37.011460] Modules linked in:
[   37.014598] CPU: 0 PID: 1 Comm: systemd-shutdow Not tainted 4.11.0-rc1-00003-g1e248b60a39b-dirty #506
[   37.023903] Hardware name: (null) (DT)
[   37.027734] task: ffff800902068000 task.stack: ffff800902064000
[   37.033739] PC is at 0x0
[...