storage/posix: add list xattr capability to lookup

BUG: 1078061
Change-Id: Ie26d28b8a74aa0d1eceff14a84c3cd3e302dcdb5
Signed-off-by: Pranith Kumar K <pkarampu@redhat.com>
Reviewed-on: http://review.gluster.org/7293
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Anand Avati <avati@redhat.com>