[virtio-serial] fix host notification when msi is disabled.
[virtio-serial] fix host notification when msi is disabled.

Related to commit 8fe2811b. If msi was not enabled (e.g. Windows
XP), an invalid interrupt object was used when sending notification
that the host is connected.

YanVugenfirer/virtio-win-arm,daynix/kvm-guest-drivers-windows,ladipro/kvm-guest-drivers-windows,vrozenfe/kvm-guest-drivers-windows,ladipro/kvm-guest-drivers-windows,gnif/kvm-guest-drivers-windows,YanVugenfirer/kvm-guest-drivers-windows,daynix/kvm-guest-drivers-windows,YanVugenfirer/kvm-guest-drivers-windows,GoogleCloudPlatform/compute-windows-drivers,YanVugenfirer/virtio-win-arm,gnif/kvm-guest-drivers-windows,YanVugenfirer/kvm-guest-drivers-windows,virtio-win/kvm-guest-drivers-windows,virtio-win/kvm-guest-drivers-windows,vrozenfe/kvm-guest-drivers-windows,YanVugenfirer/virtio-win-arm,daynix/kvm-guest-drivers-windows,ladipro/kvm-guest-drivers-windows,gnif/kvm-guest-drivers-windows,YanVugenfirer/virtio-win-arm,gnif/kvm-guest-drivers-windows,daynix/kvm-guest-drivers-windows,virtio-win/kvm-guest-drivers-windows,vrozenfe/kvm-guest-drivers-windows,virtio-win/kvm-guest-drivers-windows,YanVugenfirer/kvm-guest-drivers-windows,GoogleCloudPlatform/compute-windows-drivers,daynix/kvm-guest-drivers-windows,vrozenfe/kvm-guest-drivers-windows,GoogleCloudPlatform/compute-windows-drivers,virtio-win/kvm-guest-drivers-windows,ladipro/kvm-guest-drivers-windows,vrozenfe/kvm-guest-drivers-windows,GoogleCloudPlatform/compute-windows-drivers,YanVugenfirer/kvm-guest-drivers-windows