ZTS: devices_001_pos and devices_002_neg

Update the devices_001_pos and devices_002_neg test cases such that the
special block device file created is backed by a ZFS volume. Specifying
a specific device allows the major and minor numbers to be easily
determined. Furthermore, this avoids the potentially dangerous behavior
of opening the first block device we happen to find under /dev/.

Reviewed-by: Ryan Moeller <ryan@ixsystems.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #9773