Use /run/media/$USER instead of $XDG_RUNTIME_DIR/media

This is because of security concerns - it is way too dangerous to let
a system-daemon such as udisks manage directories in a user-controlled
location such as $XDG_RUNTIME_DIR. So now udisks2 is using
/run/media/$USER instead, see

 http://cgit.freedesktop.org/udisks/commit/?id=aa02e5fc53efdeaf66047d2ad437ed543178965b

These bugs are related

 https://bugzilla.gnome.org/show_bug.cgi?id=669797
 https://bugzilla.gnome.org/show_bug.cgi?id=646391



Signed-off-by: David Zeuthen <davidz@redhat.com>