utils_misc.virt_test_assistant: Tweaks and reorganization

Now setup log goes to main debug log, setup phase is
displayed as a test. Also, there was a bug on the
uncompress stage in case the image was already
uncompressed, fixed by passing the right flags to 7za.
Signed-off-by: NLucas Meneghel Rodrigues <lmr@redhat.com>