s390/virtio-ccw: Fix setup_vq error handling.

virtio_ccw_setup_vq() failed to unwind correctly on errors. In
particular, it failed to delete the virtqueue on errors, leading to
list corruption when virtio_ccw_del_vqs() iterated over a virtqueue
that had not been added to the vcdev's list.

Fix this with redoing the error unwinding in virtio_ccw_setup_vq(),
using a single path for all errors.

Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Reviewed-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Gleb Natapov <gleb@redhat.com>