libxl: fix attaching net device of type hostdev
    
    Chunyan sent a correct patch to fix a resource leak on error in
    libxlDomainAttachNetDevice
    
    https://www.redhat.com/archives/libvir-list/2016-March/msg00924.html
    
    I made what was thought to be an improvement and pushed the patch as
    commit e6336442. As it turns out, my change broke adding net devices
    that are actually hostdevs to the list of nets in virDomainDef. This
    patch changes e6336442 to resemble Chunyan's original, correct
    patch.