xen: Fix PV-on-HVM

In the context of PV-on-HVM under Xen, the emulated nics are supposed to be
unplug before the guest drivers are initialized, when the guest write to a
specific IO port.

Without this patch, the guest end up with two nics with the same MAC, the
emulated nic and the PV nic.

Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
