torture: Make kvm-again.sh use "scenarios" rather than "batches" file

This commit saves a few lines of code by making kvm-again.sh use the
"scenarios" file rather than the "batches" file, both of which are
generated by kvm.sh.

This results in a break point because new versions of kvm-again.sh cannot
handle "res" directories produced by old versions of kvm.sh, which lack
the "scenarios" file.  In the unlikely event that this becomes a problem,
a trivial script suffices to convert the "batches" file to a "scenarios"
file, and this script may be easily extracted from kvm.sh.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>