KVM: arm64: Add arch vcpu ioctl hook

To enable arm64-specific vcpu ioctls to be added cleanly, this
patch adds a kvm_arm_arch_vcpu_ioctl() hook so that these don't
pollute the common code.

No functional change: the -EINVAL return for unknown ioctls is
retained, though it may or may not be intentional (KVM returns
-ENXIO in various other similar contexts).

Signed-off-by: Dave Martin <Dave.Martin@arm.com>