virtio: release virtio index when fail to device_register

commit e60ea67bb60459b95a50a156296041a13e0e380e upstream.

index can be reused by other virtio device.

Signed-off-by: weiping zhang <zhangweiping@didichuxing.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>