torture: Make kvm.sh select per-scenario affinity masks

This commit causes kvm.sh to use the new kvm-assign-cpus.sh and
kvm-get-cpus-script.sh scripts to create a TORTURE_AFFINITY environment
variable containing either an empty string (for no affinity) or a list
of CPUs to pin the scenario's vCPUs to. A later commit will make
use of this information to actually pin the vCPUs.

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