Get device-by-path by looking for it instead of assuming 3rd entry.

On some systems (virtual machines so far) the device-by-path entry
from udevadm is not always in the same spot so instead actually
look for the right output instead of blindy assuming that its a
specific field in the output.

Signed-off-by: Sandon Van Ness <sandon@inktank.com>
Reviewed-by: Gary Lowell  <gary.lowell@inktank.com>