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 gvfs!68 (merged)
Closes: #856