rcutorture: Add --bootargs parameter to kvm-again.sh

The kvm-again.sh script can be used to repeat short boot-time tests,
but the kernel boot arguments cannot be changed. This means that every
change in kernel boot arguments currently necessitates a kernel build,
which greatly increases the duration of kernel-boot testing.

This commit therefore adds a --bootargs parameter to kvm-again.sh,
which allows a given kernel to be repeatedly booted, but overriding
old and adding new kernel boot parameters. This allows an old kernel
to be booted with new kernel boot parameters, avoiding the overhead of
rebuilding the kernel under test.

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

torture: Consolidate qemu-cmd duration editing into kvm-transform.sh

Currently, kvm-again.sh updates the duration in the "seconds=" comment
in the qemu-cmd file, but kvm-transform.sh updates the duration in the
actual qemu command arguments. This is an accident waiting to happen.

This commit therefore consolidates these updates into kvm-transform.sh.

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

torture: Make kvm-transform.sh update jitter commands

When rerunning an old run using kvm-again.sh, the jitter commands
will re-use the original "res" directory. This works, but is clearly
an accident waiting to happen. And this accident will happen with
remote runs, where the original directory lives on some other system.
This commit therefore updates the qemu-cmd commands to use the new res
directory created for this specific run.

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

torture: Add --duration argument to kvm-again.sh

This commit adds a --duration argument to kvm-again.sh to allow the user
to override the --duration specified for the original kvm.sh run.

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

torture: Add kvm-tranform.sh script for qemu-cmd files

This commit adds a script that transforms qemu-cmd files to allow them
and the corresponding kernels to be run in contexts other than the one
that they were created for, including on systems other than the one that
they were built on. For example, this allows the build products from a
--buildonly run to be transformed to allow distributed rcutorture testing.

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