libotutil: Add API to create an "ay" GVariant from GBytes

We used to have a version of this, but since I'm trying to use
GBytes more, this became a more common operation, and it's annoying
to type out the whole G_VARIANT_TYPE ("ay") each time, and pass
TRUE for trusted.

https://bugzilla.gnome.org/show_bug.cgi?id=706031