Use 'drive-multidisk' instead of 'gdu-enclosure' for RAID arrays

Use 'drive-multidisk' instead of 'gdu-enclosure' for RAID arrays

The 'gdu-enclosure' icon used to be in gnome-disk-utility which was
never right as it meant that gnome-disk-utility had to be installed in
order for the GIcon in the UDisksObjectInfo to make sense.

The 'drive-multidisk' icon was *just* added to gnome-icon-theme, see

 https://bugzilla.gnome.org/show_bug.cgi?id=684406
 http://git.gnome.org/browse/gnome-icon-theme/commit/?id=5644192d3f034ae614cddef2720f95408655dab3

See

 http://people.freedesktop.org/~david/gnome-disks-new-raid-icon.png

for its use in Disks.

Signed-off-by: David Zeuthen <zeuthen@gmail.com>