hw/arm: xenpvh: Add support for SMP guests

hw/arm: xenpvh: Add support for SMP guests

Add SMP support for Xen PVH ARM guests.
Create ms->smp.max_cpus ioreq servers to handle hotplug.

Note that ms->smp.max_cpus will be passed to us by the
user (Xen tools) set to the guests maxvcpus.

The value in mc->max_cpus is an absolute maximum for the
-smp option and won't be used to setup ioreq servers unless
the user explicitly specifies it with -smp.

If the user doesn't pass -smp on the command-line, smp.cpus
and smp.max_cpus will default to 1.

Signed-off-by: Edgar E. Iglesias <edgar.iglesias@amd.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>