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: Eric Blake <email@example.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <firstname.lastname@example.org>