interface: don't error out if a bond has no interfaces
    
    It's not a problem at all and causes virt-manager to break down.
    
    Note: netcf 0.2.8 and earlier generates invalid XML for a bond with no
    interfaces anyway, so in that case this error in libvirt is never
    reached since we fail earlier.
    
    Signed-off-by: Lubomir Rintel <lkundrak@v3.sk>