MAINTAINERS: Add check-qom-{interface,proplist} to QOM

Add the QOM unit tests to the QOM maintenance area so that maintainers
get CC'ed on changes and to document QOM test coverage.

Reviewed-by: Daniel P. Berrange <berrange@redhat.com>
Reviewed-by: Igor Mammedov <imammedo@redhat.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>
