vdev_id fails to handle complex device topologies

While expanding positional parameters shell requires non-single
digits to be enclosed in braces. When the SAS topology is
non-trivial the number of positional parameters generated internally
by vdev_id script (using set -- ...) easily crosses single digit limit
and vdev_id fails to generate links.

Signed-off-by: Ned Bass <bass6@llnl.gov>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #1119