LU-13152 llapi: llapi_layout_get_by_xattr groks DoM

LU-13152 llapi: llapi_layout_get_by_xattr groks DoM

llapi_layout_get_by_xattr() function must be updated to handle
lov component with LOV_PATTERN_MDT pattern.

Signed-off-by: Clement Barthelemy <clement.barthelemy@nextino.eu>
Signed-off-by: Sebastien Buisson <sbuisson@ddn.com>
Change-Id: I6553e66cd4f3b5acc65790da94555350c98fe179
Reviewed-on: https://review.whamcloud.com/37269
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Jian Yu <yujian@whamcloud.com>