libblkid: Detect external XFS log device

Detects external XFS log devices with a minimum size of 10MB.  It's possible to
craft a smaller log, but that would be very unlikely; it'd require a small XFS
filesystem and an intentionally small log, which would defeat the performance
goal of an external log in the first place.

[kzak@redhat.com: - use UUID_LOG variable name for the log uuid]

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Karel Zak <kzak@redhat.com>