qemu-io-cmds: assert that we don't have .perm requested in no-blk case

Coverity thinks blk may be NULL. It's a false-positive, as described in
a new comment.

Fixes: Coverity CID 1453194
Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <20210519090532.3753-1-vsementsov@virtuozzo.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>