installed-media: Refactor constructor a bit

Put OS guessing/detection from filename into a separate method.
os-db: More specific error for DB load failure
os-db: Add list_os_by_distro method

Add a method to get a list of OS entries given a distro name.
Optionally, caller can pass a GLib.Date to only list OSes released (or
to be released) after a particular date.
Require libosinfo >= 0.2.9
installed-media: Generic mapping of image to gnome release

Using the date on gnome-continuous image filenames, ensure that we map
the image to the first gnome release after the date on the image.
Mapping for older images will still have to be hardcoded I'm afraid.