OpenZFS 7001 - zvol_misc tests should not depend on /sbin or /usr/sbin being in PATH

OpenZFS 7001 - zvol_misc tests should not depend on /sbin or /usr/sbin being in PATH

Authored by: Hans Rosenfeld <hans.rosenfeld@nexenta.com>
Reviewed by: Toomas Soome <tsoome@me.com>
Reviewed by: Igor Kozhukhov <ikozhukhov@gmail.com>
Approved by: Dan McDonald <danmcd@omniti.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Ported-by: George Melikov <mail@gmelikov.ru>
OpenZFS-issue: https://www.illumos.org/issues/7001
OpenZFS-commit: https://github.com/openzfs/openzfs/commit/258c9c7
Closes #5801

Porting notes:
- Most of the changes were previously applied.