KVM: arm64: timers: Abstract per-timer IRQ access

As we are about to move the location of the per-timer IRQ into
the VM structure, abstract the location of the IRQ behind an
accessor. This will make the repainting sligntly less painful.

Reviewed-by: Colton Lewis <coltonlewis@google.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20230330174800.2677007-11-maz@kernel.org