torture: Parallelize kvm-series.sh guest-OS execution

torture: Parallelize kvm-series.sh guest-OS execution

Currently, kvm-series.sh builds and runs serially, which makes for
long execution times.  This commit changes its logic to build all of
the needed kernels serially, but then run the corresponding guest OSes
concurrently in batches using the entire machine.  On large systems,
this results in order-of-magnitude speedups of the guest-OS execution
portion of the runtime.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Boqun Feng <boqun.feng@gmail.com>