arm/arm64: KVM: arch_timer: Initialize cntvoff at kvm_init

Initialize the cntvoff at kvm_init_vm time, not before running the VCPUs
at the first time because that will overwrite any potentially restored
values from user space.

Cc: Andre Przywara <andre.przywara@linaro.org>
Acked-by: Marc Zynger <marc.zyngier@arm.com>
Signed-off-by: Christoffer Dall <christoffer.dall@linaro.org>