pci: Simplify PciDevice trait

There's no need for assign_irq() or assign_msix() functions from the
PciDevice trait, as we can see it's never used anywhere in the codebase.
That's why it's better to remove these methods from the trait, and
slightly adapt the existing code.

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

vmm/src/device_manager.rs

@@ -1473,8 +1473,6 @@ impl DeviceManager {
                 .map_err(DeviceManagerError::RegisterIoevent)?;
         }
 
-        virtio_pci_device.assign_msix();
-
         let virtio_pci_device = Arc::new(Mutex::new(virtio_pci_device));
 
         pci.add_device(virtio_pci_device.clone())