vm-virtio: Modify VirtioInterrupt callback into a trait

Callbacks are not the most Rust idiomatic way of programming. The right
way is to use a Trait to provide multiple implementation of the same
interface.

Additionally, a Trait will allow for multiple functions to be defined
while using callbacks means that a new callback must be introduced for
each new function we want to add.

For these two reasons, the current commit modifies the existing
VirtioInterrupt callback into a Trait of the same name.

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

pci/src/device.rs

@@ -67,7 +67,7 @@ pub trait PciDevice: BusDevice {
     }
 
     /// Assign MSI-X to this device.
-    fn assign_msix(&mut self, _msi_cb: Arc<InterruptDelivery>) {}
+    fn assign_msix(&mut self) {}
 
     /// Allocates the needed PCI BARs space using the `allocate` function which takes a size and
     /// returns an address. Returns a Vec of (GuestAddress, GuestUsize) tuples.