torture: Record jitter start/stop commands

Distributed runs of rcutorture will need to start and stop jittering on
the remote hosts, which means that the commands must be communicated to
those hosts.  The commit therefore causes kvm.sh to place these commands
in new TORTURE_JITTER_START and TORTURE_JITTER_STOP environment variables
to communicate them to the scripts that will set this up.  In addition,
this commit causes kvm-test-1-run.sh to append these commands to each
generated qemu-cmd file, which allows any remotely executing script to
extract the needed commands from this file.

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