vm-virtio: initialize MSI-X vectors with VIRTIO_MSI_NO_VECTOR

In case of the virtio frontend driver doesn't need interupts for
certain queue event, it may explicitly write VIRTIO_MSI_NO_VECTOR
to the virtio common configuration, or it may doesn't configure
the event type vector at all.

This patch initializes both MSI-X configuration vector and queue vector
with VIRTIO_MSI_NO_VECTOR, so that the backend drivers won't trigger
unexpected interrupts to the guest.

Signed-off-by: Zide Chen <zide.chen@intel.com>

virtio-devices/src/lib.rs

@@ -75,8 +75,6 @@ const VIRTIO_F_VERSION_1: u32 = 32;
 const VIRTIO_F_IOMMU_PLATFORM: u32 = 33;
 const VIRTIO_F_IN_ORDER: u32 = 35;
 
-const VIRTIO_MSI_NO_VECTOR: u16 = 0xffff;
-
 #[derive(Debug)]
 pub enum ActivateError {
     EpollCtl(std::io::Error),