sanitycheck: need to wait for harness to process QEMU outputs

There is a potential that QEMUHandler.handle() returns before
the testing harness finishes processing all the output from QEMU.
Simply wait for the harness thread to finish before returning.

Also, fix the return code in the debug message as it should be
the return code from Popen().

Signed-off-by: Daniel Leung <daniel.leung@intel.com>