libblkid: don't use UUID= for xfs external log

libblkid: don't use UUID= for xfs external log

The log UUID is usually the same as filesystem UUID, it is bad idea to
have two devices with the same UUID in the system. It seems batter to
use only LOGUUID= for xfs external log.

 # mkfs.xfs -l logdev=/dev/sdb1 /dev/sdb2

 # blkid -p -o udev /dev/sdb1 | grep FS
 ID_FS_LOGUUID=4063d453-5c01-46ce-b92f-9523d64aecc7
 ID_FS_TYPE=xfs_external_log
 ID_FS_USAGE=other

 # blkid -p -o udev /dev/sdb2 | grep FS
 ID_FS_UUID=4063d453-5c01-46ce-b92f-9523d64aecc7
 ID_FS_TYPE=xfs
 ID_FS_USAGE=filesystem

The filesystem (sdb2) uses UUID= and log (sdb1) uses LOGUUID=.

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