i386/xen: advertise XEN_HVM_CPUID_UPCALL_VECTOR in CPUID

This will allow Linux guests (since v6.0) to use the per-vCPU upcall
vector delivered as MSI through the local APIC.

Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Reviewed-by: Paul Durrant <paul@xen.org>

target/i386/kvm/kvm.c

@@ -1837,6 +1837,10 @@ int kvm_arch_init_vcpu(CPUState *cs)
                 c->eax |= XEN_HVM_CPUID_VCPU_ID_PRESENT;
                 c->ebx = cs->cpu_index;
             }
+
+            if (cs->kvm_state->xen_version >= XEN_VERSION(4, 17)) {
+                c->eax |= XEN_HVM_CPUID_UPCALL_VECTOR;
+            }
         }
 
         r = kvm_xen_init_vcpu(cs);