kvm: require KVM_CAP_SIGNAL_MSI

This was introduced in KVM in Linux 3.5, we can require it unconditionally
in kvm_irqchip_send_msi().  However, not all architectures have to implement
it so check it only in x86, the only architecture that ever had MSI injection
but not KVM_CAP_SIGNAL_MSI.

ARM uses it to detect the presence of the ITS emulation in the kernel,
introduced in Linux 4.8.  Assume that it's there and possibly fail when
realizing the arm-its-kvm device.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

target/i386/kvm/kvm.c

@@ -91,6 +91,7 @@ const KVMCapabilityInfo kvm_arch_required_capabilities[] = {
     KVM_CAP_INFO(SET_TSS_ADDR),
     KVM_CAP_INFO(EXT_CPUID),
     KVM_CAP_INFO(MP_STATE),
+    KVM_CAP_INFO(SIGNAL_MSI),
     KVM_CAP_LAST_INFO
 };