media-manager: Fix remote disk images not working

Creating VM from a remote ISO would fail silently as gnome-boxes would
only check the image's content-type, when only the fast-content-type is
usually provided.

Example error:
index-page.vala:181: Failed to create installer media for path '/run/user/1000/gvfs/smb-share:server=diskstation.local,share=test/ISOs/Fedora-Workstation-Live-x86_64-36-1.5.iso': Medium is not supported

Fetch both the content-type and the fast-content-type to detect the
image type.

See also https://gitlab.gnome.org/GNOME/gvfs/-/merge_requests/68

Closes: #856