virtio_mmio: add cleanup for virtio_mmio_remove

cleanup all resource allocated by virtio_mmio_probe.

Signed-off-by: weiping zhang <zhangweiping@didichuxing.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>