acpi: Fix suspend with Xen PV

In the Linux kernel, the following vulnerability has been resolved:

acpi: Fix suspend with Xen PV

Commit f1e525009493 ("x86/boot: Skip realmode init code when running as
Xen PV guest") missed one code path accessing real_mode_header, leading
to dereferencing NULL when suspending the system under Xen:

[  348.284004] PM: suspend entry (deep)
[  348.289532] Filesystems sync: 0.005 seconds
[  348.291545] Freezing user space processes ... (elapsed 0.000 seconds) done.
[  348.292457] OOM killer disabled.
[  348.292462] Freezing remaining freezable tasks ... (elapsed 0.104 seconds) done.
[  348.396612] printk: Suspending console(s) (use no_console_suspend to debug)
[  348.749228] PM: suspend devices took 0.352 seconds
[  348.769713] ACPI: EC: interrupt blocked
[  348.816077] BUG: kernel NULL pointer dereference, address: 000000000000001c
[  348.816080] #PF: supervisor read access in kernel mode
[  348.816081] #PF: error_code(0x0000) - not-present page
[  348.816083] PGD 0 P4D 0
[  348.816086] Oops: 0000 [#1] PREEMPT SMP NOPTI
[  348.816089] CPU: 0 PID: 6764 Comm: systemd-sleep Not tainted 6.1.3-1.fc32.qubes.x86_64 #1
[  348.816092] Hardware name: Star Labs StarBook/StarBook, BIOS 8.01 07/03/2022
[  348.816093] RIP: e030:acpi_get_wakeup_address+0xc/0x20

Fix that by adding an optional acpi callback allowing to skip setting
the wakeup address, as in the Xen PV case this will be handled by the
hypervisor anyway.