dump: Move HMP command handlers to dump/

dump: Move HMP command handlers to dump/

Move the HMP handlers related to qapi/dump.json to
dump/dump-hmp-cmds.c, where they are covered by MAINTAINERS section
"Dump", just like qapi/dump.json.

Cc: Marc-André Lureau <marcandre.lureau@redhat.com>
Cc: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20190619201050.19040-18-armbru@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
[Commit message typo fixed]