Revert "libotutil: Make use of GBytes in ot_variant_read()"
Revert "libotutil: Make use of GBytes in ot_variant_read()"

This reverts commit c77908bf514d61e75798932f61b5b414d9e36a3c; we can't
do this since g_variant_get_data_as_bytes() is a GLib 2.36 API.

sujitfulse/ostree,gatispaeglis/ostree,resin-io/ostree,gatispaeglis/ostree,leandrosansilva/ostree,gatispaeglis/ostree,leandrosansilva/ostree,GNOME/ostree,resin-io/ostree,GNOME/ostree,GNOME/ostree,GNOME/ostree,sujitfulse/ostree,resin-io/ostree,gatispaeglis/ostree,gatispaeglis/ostree,gatispaeglis/ostree,leandrosansilva/ostree,sujitfulse/ostree,resin-io/ostree,leandrosansilva/ostree,sujitfulse/ostree,resin-io/ostree,GNOME/ostree,sujitfulse/ostree,leandrosansilva/ostree