libvirt.test: Fix problem in virsh iface test

1.Skip OSError when delete a bridge
2.Filter 'net_restart' cases because it's dangerous

Signed-off-by: Li Yang <liyang.fnst@cn.fujitsu.com>
Merge pull request #237 from liyangfnst/iface_fix

libvirt.test: Fix problem in virsh iface test