vmtests: validate formatting/mounting of a disk without a partition table
Several use-cases involve formatting and entire disk with a filesystem.
Exercise this code by adding additional disks and tests to the test_basic
test. Utilize the btrfs filesystem type. Adding this test exposed several
issues with both fstab writing (it didn't know what to do with volumes of
type 'disk') and typo's in postinstall code which installed filesystem
utilities into the target (brtfs vs. btrfs). A couple other minor fixes
were included to ensure the testsuite