If error occurs while processing the virtio request we should call
'virtqueue_detach_element' to detach the element from the virtqueue
before free the elem.

Signed-off-by: Li Qiang <liq3ea@163.com>
Message-Id: <20200816142245.17556-1-liq3ea@163.com>
Fixes: 910b2576 ("virtio-mem: Paravirtualized memory hot(un)plug")
Acked-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>