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: <20200813165125.59928-1-liq3ea@163.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Fixes: 5f503cd9 ("virtio-pmem: add virtio device")
Reviewed-by: Pankaj Gupta <pankaj.gupta.linux@gmail.com>