The next commit will move a caller of help_cmd() to a new file.
Including monitor/monitor-internal.h there just for help_cmd() feels
silly.  Better to provide it in monitor/hmp.h suitably renamed.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20230124121946.1139465-8-armbru@redhat.com>