vmm: Add MSI-X support to virtio-pci devices

In order to allow virtio-pci devices to use MSI-X messages instead
of legacy pin based interrupts, this patch implements the MSI-X
support for cloud-hypervisor. The VMM code and virtio-pci bits have
been modified based on the "msix" module previously added to the pci
crate.

Fixes #12

Signed-off-by: Sebastien Boeuf <sebastien.boeuf@intel.com>

vm-virtio/src/queue.rs

@@ -218,6 +218,9 @@ pub struct Queue {
     /// Inidcates if the queue is finished with configuration
     pub ready: bool,
 
+    /// Interrupt vector index of the queue
+    pub vector: u16,
+
     /// Guest physical address of the descriptor table
     pub desc_table: GuestAddress,
 
@@ -238,6 +241,7 @@ impl Queue {
             max_size,
             size: max_size,
             ready: false,
+            vector: 0,
             desc_table: GuestAddress(0),
             avail_ring: GuestAddress(0),
             used_ring: GuestAddress(0),