fence: Add network interface select option for fence_xvmd
    
     1. fence_xvmd selects wrong network interface to listen on if host has
        multiple interfaces and target interface is not for default route.
        As a result, fence_xvmd does not repond to fence_xvm's request.
     2. fence_xvmd cannot start if default route is not set.
    
     Ex: fence_xvmd -I <interface_name>
    
    Signed-Off-By: Satoru SATOH <satoru.satoh@gmail.com>
    Signed-Off-By: Lon Hohberger <lhh@redhat.com>