x86/xen: Provide a "Xen PV" APIC driver to support >255 VCPUs

Instead of mangling the default APIC driver, provide a Xen PV guest
specific one that explicitly provides appropriate methods.

This allows use to report that all APIC IDs are valid, allowing dom0
to boot with more than 255 VCPUs.

Since the probe order of APIC drivers is link dependent, we add in an
late probe function to change to the Xen PV if it hadn't been done
during bootup.

Suggested-by: David Vrabel <david.vrabel@citrix.com>
Reported-by: Cathy Avery <cathy.avery@oracle.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Signed-off-by: David Vrabel <david.vrabel@citrix.com>