KVM: arm64: Restructure the point where has_run_once is advertised

Restructure kvm_vcpu_first_run_init() to set the has_run_once
flag after having completed all the "run once" activities.

This includes moving the flip of the userspace irqchip static key
to a point where nothing can fail.

Reviewed-by: Andrew Jones <drjones@redhat.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>