qom: Move QMP command handlers to qom/

qom: Move QMP command handlers to qom/

The handlers for qapi/qom.json's QMP commands are in
monitor/qmp-cmds.c.  Move them to new qom/qom-qmp-cmds.c, where they
are covered by MAINTAINERS section QOM, just like qapi/qom.json.

Move along qmp_device_list_properties() even though it's specified in
qapi/qdev.json, because it's so similar to qmp_qom_list_properties().

Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: "Daniel P. Berrange" <berrange@redhat.com>
Cc: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20190619201050.19040-6-armbru@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>