torture: Check for nul bytes in console output

When starting a new torture run while an old one is still running, both
qemu processes can be outputting to the same console.out file.  This can
cause quite a bit of confusion, so this commit checks for this situation,
which is normally indicated by nul bytes in the console output.  Yes,
if your new run uses up an exact number of blocks of the file, this
check will be ineffective, but the odds are not bad.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Pranith Kumar <bobby.prani@gmail.com>