rcutorture: Tweak kvm options

rcutorture: Tweak kvm options

In one of my rcutorture tests the TSC clocksource got marked unstable
due to a large difference in the TSC value. I'm not sure if the guest
run for a long time with disabled interrupts or if the host was very
busy and didn't schedule the guest for some time.

I took a look on the qemu/KVM options and decided to update the options:

- Use kvm{32|64} as CPU. We could probably use `host' (like ARM does)
  for maximum available features but since we don't run any userland I'm
  not sure if it makes any difference.

- Drop the "noapic" option. There is no history why the APIC was disabled,
  I see no reason for it.  Once old qemu versions fade away, we can add
  "x2apic=on,tsc-deadline=on,hypervisor=on,tsc_adjust=on".

- Additional config options. It ensures that the kernel knowns that it
  runs as a kvm guest and can use virt devices like the kvm-clock as
  clocksource. The kvm-clock was the main motivation here.

- I didn't add a random HW device. It would make the random device ready
  earlier (not it doesn't complete the initialisation at all) but I
  doubt that there is any need for this.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
[ paulmck: The world is not quite ready for CONFIG_PARAVIRT_SPINLOCKS=y
  and x2apic, so they are omitted for the time being. ]
Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>