KVM: x86/xen: Add KVM_XEN_VCPU_ATTR_TYPE_VCPU_ID

In order to intercept hypercalls such as VCPUOP_set_singleshot_timer, we
need to be aware of the Xen CPU numbering.

This looks a lot like the Hyper-V handling of vpidx, for obvious reasons.

Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <20220303154127.202856-12-dwmw2@infradead.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>