x86: Skip check apic_id_limit for Xen

Xen vIOMMU device model will be in Xen hypervisor. Skip vIOMMU
check for Xen here when vcpu number is more than 255.

Signed-off-by: Lan Tianyu <tianyu.lan@intel.com>
Message-Id: <1502842933-8323-1-git-send-email-tianyu.lan@intel.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
