service: Ban subdirs in logfile name

There are two reasons to ban subdirs in logfile name.
First, process might be in different namespace, so it is right to give us fd
for work dir, just like we did with images dir. Second, as service might be ran
as root, it is unsafe to give an opportunity to fill any dir with trash.
If you wan't to put logs/stats somwhere else than images_dir, you could
set work_dir_fd.

Signed-off-by: Ruslan Kuprieiev <kupruser@gmail.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>