img: Warn about absolute paths for parent images dir

When working with mntns, the absolute path in parent symlink will
not be open-able on restore. However, completely banning this case
is not good.

Affects #116

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>