vm-configurator: Ensure we use best architecture

This fixes the issue of Boxes picking-up 'i686' architecture if
architecture of installer media is unknown. Boxes will now pick 'x86_64'
in such situation.

https://bugzilla.gnome.org/show_bug.cgi?id=720800
installed-media: Throw more specific error

For unsupported file, throw a more specific error.

https://bugzilla.gnome.org/show_bug.cgi?id=720801
media-manager: More reliable check for media type

Instead of checking for mime-types on file to see if its an
installer/live media, we now rely on the check for installed media based
on file extension.

This fixes the regression from commit d231c27 which completely broke
handling of ISO9660 devices (loop-mounted ISOs and hardware).

https://bugzilla.gnome.org/show_bug.cgi?id=720801
Revert "util: Add is_mime_type()"

This reverts commit a9e6af5fa88c3748bbf748f68450eea95dd2b3b8.

We don't need this utility function anymore.

https://bugzilla.gnome.org/show_bug.cgi?id=720801
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