qapi: Split qom.json and qdev.json off misc.json

Move commands object-add, object-del, qom-get, qom-list,
qom-list-properties, qom-list-types, and qom-set with their types from
misc.json to new qom.json.

Move commands device-list-properties, device_add, device-del, and
event DEVICE_DELETED from misc.json to new qdev.json.

Add both new files to MAINTAINERS section QOM.

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: <20190619201050.19040-5-armbru@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
[Straightforwardly updated for "MAINTAINERS: Make section "QOM" cover
qdev as well"]