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>