virtio: use err label in __vring_new_virtqueue()

Using error label for unwind in __vring_new_virtqueue. This is useful
for future refacotring.

Signed-off-by: Jason Wang <jasowang@redhat.com>
Link: https://lore.kernel.org/r/20210604055350.58753-7-jasowang@redhat.com


Signed-off-by: Michael S. Tsirkin <mst@redhat.com>