ZTS: clean_mirror and scrub_mirror cleanup

Remove the dependency on partitionable devices for the clean_mirror
and scrub_mirror test cases. This allows for the setup and cleanup
of the test cases to be simplified by removing the need for complex
partitioning.

This change also resolves a issue where the clean_mirror devices
were not being properly damaged since the device name was not a
full path. The result being loopX files were being left in the
top level test_results directory.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #7434
Closes #7690