xen: check if device is assigned to guest before reattaching

xen: check if device is assigned to guest before reattaching

Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=664059

Reattaching pci device back to host without destroying guest or
detaching device from guest would cause host to crash. This patch adds
a check before doing device reattach. If the device is being assigned
to guest, libvirt refuses to reattach device to host. The patch only
works for Xen, for it just checks xenstore to get pci device
information.

Signed-off-by: Yufang Zhang <yuzhang@redhat.com>