linux: include <bond> element for bonds with no slaves
    
    The missing element makes libvirt sad:
    
      $ ncftool dumpxml --live nm-bond
      
      <interface name="nm-bond" type="bond">
        <link state="unknown" speed="0"/>
        <protocol family="ipv4">
          <ip address="1.2.3.4" prefix="8"/>
        </protocol>
      </interface>
    
      $ virsh iface-dumpxml nm-bond
      error: XML error: bond interface misses the bond element
    
    This is analogous what was done in d32a464c (Always add <bridge> element to
    bridge if, even if no physdev is attached) for bridges.