libvirt: enable apic setting for Xen or KVM guest.

Bug 1086352

Currently, nova doesn't enable apic setting for Xen or KVM guest
in its libvirt driver. Windows guests would not boot successful in
such case. This patch adds apic setting in libvirt driver for Xen
or KVM guest, which would fix this problem. A check is also added
to libvirt guest config test case for this patch.

(cherry picked from commit d0e930bd9f1bddf6223635fdc463b2176cc39690)

Change-Id: Ie213c9d086f77faf0cdc5a32337c5bf4b828cf5f