Backend domain generates its IP address based on frontend IP, not
settings given from dom0. So change frontend method to the same (for
DispVM it makes a difference). Now "qubes-gateway" xenstore entry is
basically primary DNS address only.
Get rid of underscores in filenames, use dashes instead.
This is first part of cleanup in filenames.
"qubes_rpc" still untouched - will be in separate commit.