Distributed execution of rcutorture is eased if the qemu execution can
be split from the building of the kernel, as this allows target systems
to be used that are not set up to build kernels.  It also avoids issues
with toolchain version skew across the cluster, aside of course from
qemu and KVM version skew.

This commit therefore records needed data as comments in the qemu-cmd file
and moves recording of the starting time to just before qemu is launched.
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>