libvirt.tests: Add test for iface related commands

This test including following commands:
```
virsh iface-list
virsh iface-define
virsh iface-dumpxml
virsh iface-start
virsh iface-mac
virsh-iface-name
virsh iface-destroy
virsh iface-undefine
```

By default, using 'lo' device to run test, user can specify another exist interface or give a XML file to define an non-exist interface for testing.

Signed-off-by: Yanbing Du <ydu@redhat.com>