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>

vm-virtio/src/lib.rs

@@ -23,6 +23,8 @@ pub use queue::*;
 pub type VirtioIommuRemapping =
     Box<dyn Fn(u64) -> std::result::Result<u64, std::io::Error> + Send + Sync>;
 
+pub const VIRTIO_MSI_NO_VECTOR: u16 = 0xffff;
+
 // Types taken from linux/virtio_ids.h
 #[derive(Copy, Clone, Debug)]
 #[allow(dead_code)]