libavb_user: Try /dev/block/by-name before looking at /etc/fstab.

Recent versions of Android have a /dev/block/by-name/ directory with
symlinks to partitions using the partition label. Use this in lieu of
the look-in-fstab-for-misc-device-path kludge currently being used.

Test: All unit tests still pass.
Test: Manually tested on a device without 'misc' entry in fstab.
Bug: None
Change-Id: Ie2225d35128ec335e65296de8d638f15ba978df4