virtio-serial: pci: Allow MSI to be disabled Michael noted we don't allow disabling of MSI for the virtio-serial-pci device. Fix that. Signed-off-by: Amit Shah <amit.shah@redhat.com> CC: "Michael S. Tsirkin" <mst@redhat.com> Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>