qcow2: Fix error path in qcow2_snapshot_load_tmp If the bdrv_read() of the snapshot's L1 table fails, return the right error code and make sure that the old L1 table is still loaded and we don't break the BlockDriverState completely. Signed-off-by: Kevin Wolf <kwolf@redhat.com> Reviewed-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>