daemon: Implement set-label for XFS and fix it for btrfs (RHBZ#986875).

Implement 'set-label' for XFS filesystems.

Fix the call for btrfs.  Previous
commit d5817537fa6c51a7f851ecc5e4e63e60609e0c03 added some bogus
documentation implying this call would work for btrfs, but it did
not.

Add tests.
New API: Implement set-uuid for ext2/3/4 and XFS (RHBZ#986877).

Also includes tests.
docs: Document labels and UUIDs in the API overview section.
sysprep: Add new fs-uuids operation.

This creates new random UUIDs for all filesystems in a guest.