Fix long scan problem when BIOS floppy setting incorrect

The call to "blkid -c /dev/null" on Precise Pangolin Alpha 1 takes
exceedingly long.  This occurs when the BIOS is incorrectly set to
indicate a floppy drive is present when none is physically installed. 
Use cached blkid results instead.

Ubuntu launchpad 910379 - Gparted does not start and continues to
                          scan devices
https://bugs.launchpad.net/ubuntu/+source/gparted/+bug/910379