qapi: Support multiple command registries per program

The command registry encapsulates a single command list.  Give the
functions using it a parameter instead.  Define suitable command lists
in monitor, guest agent and test-qmp-commands.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <1488544368-30622-6-git-send-email-armbru@redhat.com>
[Debugging turds buried]
Reviewed-by: Eric Blake <eblake@redhat.com>