xen: support large numbers of CPUs with vcpu info placement

When vcpu info placement is supported, we're not limited to MAX_VIRT_CPUS
vcpus.  However, if it isn't supported, then ignore any excess vcpus.

Signed-off-by: Mukesh Rathor <mukesh.rathor@oracle.com>
Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>