KVM: arm/arm64: vgic: Rename kvm_vgic_vcpu_init to kvm_vgic_vcpu_enable

This function really doesn't init anything, it enables the CPU
interface, so name it as such, which gives us the name to use for actual
init work later on.

Signed-off-by: Christoffer Dall <cdall@linaro.org>
Reviewed-by: Eric Auger <eric.auger@redhat.com>