KVM: arm64: Merge kvm_arch_vcpu_run_pid_change() and kvm_vcpu_first_run_init()

The kvm_arch_vcpu_run_pid_change() helper gets called on each PID
change. The kvm_vcpu_first_run_init() helper gets run on the...
first run(!) of a vcpu.

As it turns out, the first run of a vcpu also triggers a PID change
event (vcpu->pid is initially NULL).

Use this property to merge these two helpers and get rid of another
arm64-specific oddity.

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