avoid shelling to blkid

[alexeldeib]

This line matches on output instead of the success of the command because I struggled to differentiate blkid itself failing, and blkid failing to find a partition on the disk (but executing successfully).

Ideally we remove any std::process::Command stuff, and read from one of several sources (not sure where the right data is):

• /sys/block/*
• /dev/disk/by-uuid -> read symlinks to /dev/sdxx
• /etc/mtab -> partition data