torture: Make kvm-test-1-run-qemu.sh apply affinity

This commit causes the kvm-test-1-run-qemu.sh script to check the
TORTURE_AFFINITY environment variable and to add "taskset" commands to
the qemu-cmd file.  The first "taskset" command is applied only if the
TORTURE_AFFINITY environment variable is a non-empty string, and this
command pins the current scenario's guest OS to the specified CPUs.
The second "taskset" command reports the guest OS's affinity in a new
"qemu-affinity" file.

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