xen: Fix, no unplug of pt device by platform device. 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>