vdpa_sim_blk: Fix the potential leak of mgmt_dev

If the shared_buffer allocation fails, need to unregister mgmt_dev first.

Cc: stable@vger.kernel.org
Fixes: abebb162

 ("vdpa_sim_blk: support shared backend")
Signed-off-by: Shawn.Shao <shawn.shao@jaguarmicro.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Message-Id: <20230821060333.1155-1-shawn.shao@jaguarmicro.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>