torture: Allow kvm.sh --datestamp to specify subdirectories

Scripts like kvm-check-branches.sh group runs under a single directory
in resdir in order to allow easier retrospective analysis.  However, they
do this by letting kvm.sh create a directory as usual and then moving it
after the run.  This can be very confusing when looking at the results
while kvm-check-branches.sh is running.  This commit therefore enables
--datestamp to hand subdirectories to kvm.sh.

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