hw/arm/virt: Don't incorrectly claim architectural timer to be edge-triggered

hw/arm/virt: Don't incorrectly claim architectural timer to be edge-triggered

The architectural timers in ARM CPUs all have level triggered interrupts
(unless you're using KVM on a host kernel before 4.4, which misimplemented
them as edge-triggered).

We were incorrectly describing them in the device tree as edge triggered.
This can cause problems for guest kernels in 4.8 before rc6:
 * pre-4.8 kernels ignore the values in the DT
 * 4.8 before rc6 write the DT values to the GIC config registers
 * newer than rc6 ignore the DT and insist that the timer interrupts
   are level triggered regardless

Fix the DT so we're describing reality. For backwards-compatibility
purposes, only do this for the virt-2.9 machine onward.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Andrew Jones <drjones@redhat.com>