torture: Report diagnostics from qemu

The current script does record qemu diagnostics, but the user has to
know where to look for them.  This commit therefore puts them into the
Warnings file so that kvm-recheck.sh will display them.  This change is
especially useful if you are in the habit of killing the qemu process
when you realize that you messed something up, but then later on wonder
why the process terminated early.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>