Move hmp_device_add(), hmp_device_del() from monitor/hmp-cmds.c to
qdev-monitor.c, where they are covered by MAINTAINERS section "QOM",
just like qapi/qdev.json.  hmp_info_qtree() and hmp_info_qdm() are
already there.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20190709185936.31335-1-armbru@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Tested-by: Philippe Mathieu-Daudé <philmd@redhat.com>