LU-16501 tgt: add qos debug

LU-16501 tgt: add qos debug

Add several debug lines for QOS allocator.
Patch also changes S_CLASS subsystem to S_LOV in
lu_tgt_desc_tgt.c thus it can be enabled to capture
only QOS debugging.

Test-Parameters: trivial
Signed-off-by: Sergey Cheremencev <scherementsev@ddn.com>
Change-Id: I2dd53022d7f199e7c521bbf78acc4a8bf4abca51
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/49977
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Alex Zhuravlev <bzzz@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>