torture: Make torture.sh tolerate runs having bad kvm.sh arguments

Currently, torture.sh assumes excessive levels of reviewer competence
and thus fails to gracefully handle cases where it is tricked into giving
kvm.sh invalid arguments.  This commit therefore upgrades error handling
to more gracefully handle this situation.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Neeraj Upadhyay (AMD) <neeraj.upadhyay@kernel.org>