Eliminate illumos whole disk special case when searching for a ZFS vdev

This special case prevented locating vdevs which start with c[0-9] e.g.
gptid/c6cde092-504b-11e4-ba52-c45444453598 hence it was impossible to
online a vdev via its path.

Submitted by: Peter Xu <xzpeter@gmail.com>
MFC after: 2 weeks
Sponsored by: Multiplay