New APIs: findfs-label and findfs-uuid

These two calls wrap up the /sbin/findfs command, allowing you
to find a filesystem by only knowing its label or UUID.

This is especially useful when resolving LABEL=... or UUID=...
entries in /etc/fstab.

Sample guestfish session:

><fs> vfs-uuid /dev/vda1
277dd61c-bf34-4253-a8dc-df500a05e7df
><fs> findfs-uuid 277dd61c-bf34-4253-a8dc-df500a05e7df
/dev/vda1
><fs> vfs-label /dev/vda1
/boot
><fs> findfs-label /boot
/dev/vda1
><fs> vfs-uuid /dev/VolGroup00/LogVol00
40ce7c36-82ce-4a12-a99d-48f5e054162c
><fs> findfs-uuid 40ce7c36-82ce-4a12-a99d-48f5e054162c
/dev/mapper/VolGroup00-LogVol00
><fs> findfs-uuid 12345678
libguestfs: error: findfs_uuid: findfs: unable to resolve 'UUID=12345678'