python/qemu/machine.py: refactor _qemu_args()

 - use shorter construction
 - don't create new dict if not needed
 - drop extra unpacking key-val arguments
 - drop extra default values

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: John Snow <jsnow@redhat.com>
Message-Id: <20210824083856.17408-24-vsementsov@virtuozzo.com>
Signed-off-by: Hanna Reitz <hreitz@redhat.com>