arm64: KVM: Introduce per-vcpu kvm device controls

In some cases it needs to get/set attributes specific to a vcpu and so
needs something else than ONE_REG.

Let's copy the KVM_DEVICE approach, and define the respective ioctls
for the vcpu file descriptor.

Signed-off-by: Shannon Zhao <shannon.zhao@linaro.org>
Reviewed-by: Andrew Jones <drjones@redhat.com>
Acked-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>

include/uapi/linux/kvm.h

@@ -851,6 +851,7 @@ struct kvm_ppc_smmu_info {
 #define KVM_CAP_HYPERV_SYNIC 123
 #define KVM_CAP_S390_RI 124
 #define KVM_CAP_ARM_PMU_V3 125
+#define KVM_CAP_VCPU_ATTRIBUTES 126
 
 #ifdef KVM_CAP_IRQ_ROUTING