LU-11213 lmv: use lu_tgt_descs to manage tgts

LU-11213 lmv: use lu_tgt_descs to manage tgts

Like LOD, use lu_tgt_descs to manage tgts, so that they can
share tgt management code.

TODO: use the same tgt management code for LOV/LFSCK.

Signed-off-by: Lai Siyao <lai.siyao@whamcloud.com>
Change-Id: I843e85e0f8b05e21056567cebfea0a8fff1d4ef8
Reviewed-on: https://review.whamcloud.com/35218
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Hongchao Zhang <hongchao@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>