KVM: arm/arm64: Set VCPU affinity for virt timer irq

As we are about to take physical interrupts for the virtual timer on the
host but want to leave those active while running the VM (and let the VM
deactivate them), we need to set the vtimer PPI affinity accordingly.

Signed-off-by: Christoffer Dall <cdall@linaro.org>
Reviewed-by: Marc Zyngier <marc.zyngier@arm.com>