[Libvir] [PATCH] check the file name for --log

Hi, This patch fixes the trouble if execute virsh with virsh --log --debug. In this case FILE "--debug" is generated and --debug is off. This patch treat as error return if it uses the hyphen "-" at the head of log file name. Signed-off-by: Atsushi SAKAI <sakaia@jp.fujitsu.com> Thanks Atsushi SAKAI