util: replace virHostdevNetConfigReplace with ...(Save|Set)NetConfig()
    
    These two operations will need to be separated so that saving of the
    original config is done before detaching the host net driver, and
    setting the new config is done after attaching vfio-pci. This patch
    splits the single function into two, but for now calls them together
    (to make bisecting easier if there is a regression).