Add caller-validation on Disk.StaticDevPath

Since in objects we don't have access to utils.py, we add a warning that
the result value from objects.Disk.StaticDevPath might not be a valid
path, and change its only caller to validate the path.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>