virttest.qemu_monitor: ignore empty args in qmp's send_args_cmd()

We will meet following error:

QMPCmdError: QMP command 'system_powerdown' failed    (arguments: {'': ''},
   error message: {u'data': {u'name': u''}, u'class': u'InvalidParameter',
 u'desc': u"Invalid parameter ''"})

Signed-off-by: Feng Yang <fyang@redhat.com>