libvirt.tests: A new test case for iface-bridge/iface-unbridge

Signed-off-by: Yanbing Du <ydu@redhat.com>
Just run these these cases in my box， in case broken our local network, in .cfg:
```
iface_name = "lo" 
ping_ip = "127.0.0.1" 
bridge_option = "--no-stp"  
```
then all cases passed:
```
#./run -t libvirt --test=type_specific.ydu-libvirt.virsh.iface_bridge
Running setup. Please wait...
SETUP: PASS (20.06 s)
DATA DIR: /home/ydu/Work/virt-test/shared/data
DEBUG LOG: /home/ydu/Work/virt-test/logs/run-2014-06-16-14.42.10/debug.log
TESTS: 14
(1/14) type_specific.ydu-libvirt.virsh.iface_bridge.positive_test.no_option: PASS (12.24 s)
(2/14) type_specific.ydu-libvirt.virsh.iface_bridge.positive_test.bridge_no_stp: PASS (12.29 s)
(3/14) type_specific.ydu-libvirt.virsh.iface_bridge.positive_test.bridge_delay_1: PASS (11.78 s)
(4/14) type_specific.ydu-libvirt.virsh.iface_bridge.positive_test.bridge_delay_2: PASS (12.30 s)
(5/14) type_specific.ydu-libvirt.virsh.iface_bridge.positive_test.bridge_no_start: PASS (6.69 s)
(6/14) type_specific.ydu-libvirt.virsh.iface_bridge.positive_test.bridge_delay_no_stp: PASS (14.47 s)
(7/14) type_specific.ydu-libvirt.virsh.iface_bridge.positive_test.bridge_no_stp_no_start: PASS (6.05 s)
(8/14) type_specific.ydu-libvirt.virsh.iface_bridge.positive_test.unbridge_no_start: PASS (11.71 s)
(9/14) type_specific.ydu-libvirt.virsh.iface_bridge.negative_test.invalid_delay_1: PASS (6.41 s)
(10/14) type_specific.ydu-libvirt.virsh.iface_bridge.negative_test.invalid_delay_2: PASS (5.38 s)
(11/14) type_specific.ydu-libvirt.virsh.iface_bridge.negative_test.invalid_delay_3: PASS (5.12 s)
(12/14) type_specific.ydu-libvirt.virsh.iface_bridge.negative_test.invalid_bridge_option: PASS (5.20 s)
(13/14) type_specific.ydu-libvirt.virsh.iface_bridge.negative_test.invalid_unbridge_option: PASS (11.66 s)
(14/14) type_specific.ydu-libvirt.virsh.iface_bridge.negative_test.unbridge_iface: PASS (4.92 s)
TOTAL TIME: 126.30 s (02:06)
TESTS PASSED: 14
TESTS FAILED: 0
SUCCESS RATE: 100.00 %