The Xen platform device will unplug any NICs if requested by the guest (PVonHVM)
including a NIC that would have been passthrough. This patch makes sure that a
passthrough device will not be unplug.

Reported-by: "Zhang, Yang Z" <yang.z.zhang@intel.com>
Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>