The NBD spec, and even our code comment, says that if the client
asks for NBD_OPT_LIST_META_CONTEXT with 0 queries, then we should
reply with (a possibly-compressed representation of) ALL contexts
that we are willing to let them try.  But commit 3d068aff forgot
to advertise qemu:dirty-bitmap:FOO.
Signed-off-by: NEric Blake <eblake@redhat.com>
Message-Id: <20181130023232.3079982-2-eblake@redhat.com>
Reviewed-by: NVladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>