qapi: new qmp command nbd-server-add-bitmap

qapi: new qmp command nbd-server-add-bitmap

For now, the actual command ix x-nbd-server-add-bitmap, reflecting
the fact that we are still working on libvirt code that proves the
command works as needed, and also the fact that we may remove
bitmap-export-name (and just require that the exported name be the
bitmap name).

Signed-off-by: Vladimir Sementsov-Ogievskiy <[email protected]>
Message-Id: <20180609151758[email protected]>
Reviewed-by: Eric Blake <[email protected]>
[eblake: make the command experimental by adding x- prefix]
Signed-off-by: Eric Blake <[email protected]>