x86/xen/smp_pv: Prepare for separate mpparse callbacks

Provide a wrapper around the existing function and fill the new callbacks
in.

No functional change as the new callbacks are not yet operational.

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/20240212154639.745028043@linutronix.de