libvirt.test: Fix bugs and remove cases from detach-device

1.Remove some cases for cdrom test
2.Check if option is supported by libvirt

Signed-off-by: Li Yang <liyang.fnst@cn.fujitsu.com>
Merge pull request #253 from liyangfnst/dt_device_fix

libvirt.test: Fix bugs and remove cases from detach-device