pci: Refactor bus.rs to better fit a PCI bus's semantics

This commit refactors the PCI bus struct. It has two major focuses.
First, we change the type of `device_ids` in `PciBus` to an array. A
fixed-size array better reflects real PCI bus constraints, especially
its limited number of PCI devices. Moreover, it can't be grown
accidentally.

The second focus is changing the type of the key of `devices` in
`PciBus` to `u8`, since device IDs are not allowed to exceed 31. We
furthermore replace magic numbers with constants and make them publicly
available so we can use them in a follow-up change when parsing user
input.

Signed-off-by: Pascal Scholz <pascal.scholz@cyberus-technology.de>
On-behalf-of: SAP pascal.scholz@sap.com

vmm/src/device_manager.rs

@@ -4125,7 +4125,7 @@ impl DeviceManager {
             .unwrap();
 
         pci_bus
-            .add_device(bdf.device() as u32, pci_device)
+            .add_device(bdf.device(), pci_device)
             .map_err(DeviceManagerError::AddPciDevice)?;
 
         self.bus_devices.push(Arc::clone(&bus_device));