x86/xen/smp_pv: Register fake APICs

XENPV does not use the APIC. It's just piggy packing on the infrastructure
and fiddles with global variables as it sees fit.

These global variables are going away, so let XENPV register pseudo APIC
IDs to keep the accounting correct and keep up the illusion that XEN/PV is
something sane.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Michael Kelley <mhklinux@outlook.com>
Tested-by: Sohil Mehta <sohil.mehta@intel.com>
Link: https://lore.kernel.org/r/20240213210251.940043512@linutronix.de