Add domif-setlink and domif-getlink testing 

Tracking issue:
#240


Add virsh domif-setlink domif-getlink test
    
    Introduce virsh domif-setlink domif-getlink  testing:
    
        1. Positive testing
            1.1 Running domain and shutting off domain testing
            1.2 Setlink domain interface up and down then check
            1.3 Options "--config" testing
            1.4 Interface name and MAC address testing
    
        2. Negative testing
            2.1 Invalid options testing
            2.2 Shutting off domain with interface name
    
    Signed-off-by: whuang <whuang@redhat.com>