virtio_mmio: add cleanup for virtio_mmio_probe

As mentioned at drivers/base/core.c:
/*
 * NOTE: _Never_ directly free @dev after calling this function, even
 * if it returned an error! Always use put_device() to give up the
 * reference initialized in this function instead.
 */
so we don't free vm_dev until vm_dev.dev.release be called.

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