xen/x2apic: enable x2apic mode when supported for HVM

There's no point in disabling x2APIC mode when running as a Xen HVM
guest, just enable it when available.

Remove some unneeded wrapping around the detection functions, and
simply provide a xen_x2apic_available helper that's a wrapper around
x2apic_supported.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Link: https://lore.kernel.org/r/20220121090146.13697-1-roger.pau@citrix.com


Signed-off-by: Juergen Gross <jgross@suse.com>