hostdev: skip ACS check when using VFIO for device assignment
    
    The ACS checks are meaningless when using the more modern VFIO driver
    for device assignment since VFIO has its own more complete and exact
    checks, but I didn't realize that when I added support for VFIO. This
    patch eliminates the ACS check when preparing PCI devices for
    assignment if VFIO is being used.
    
    This resolves:
    
      https://bugzilla.redhat.com/show_bug.cgi?id=1256486