torture: Add torture.sh --guest-cpu-limit argument for limited hosts

torture: Add torture.sh --guest-cpu-limit argument for limited hosts

Some servers have limitations on the number of CPUs a given guest OS
can use.  In my earlier experience, such limitations have been at least
half of the host's CPUs, but in a recent example, this limit is less
than 40%.  This commit therefore adds a --guest-cpu-limit argument that
allows such low limits to be made known to torture.sh.

Signed-off-by: "Paul E. McKenney" <paulmck@kernel.org>
Signed-off-by: Neeraj Upadhyay <neeraj.upadhyay@kernel.org>