fs_mgr: Support filesystem labels in fstab

When mounting fstab entries it is practical to be able to specify
for example LABEL=data instead of a specific block device.

This is particularly important for the new Android Emulator code base,
which uses virtio block devices for the various partitions
(system,data,cache), because there is no defined ordering between the
way the filesystems are specified to the emulator and how they are
enumerated by the android kernel as /dev/vdX.

Change-Id: I12c3db0ba6a515dc8e917e0349afd257888d3aef
Signed-off-by: Christoffer Dall <christoffer.dall@linaro.org>