vmm: Break the cyclic dependency between DeviceManager and IO bus

By inserting the DeviceManager on the IO bus, we introduced some cyclic
dependency:

  DeviceManager ---> AddressManager ---> Bus ---> BusDevice
        ^                                             |
        |                                             |
        +---------------------------------------------+

This cycle needs to be broken by inserting a Weak reference instead of
an Arc (considered as a strong reference).

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

vmm/src/vm.rs

@@ -222,6 +222,8 @@ pub struct Vm {
     state: RwLock<VmState>,
     cpu_manager: Arc<Mutex<cpu::CpuManager>>,
     memory_manager: Arc<Mutex<MemoryManager>>,
+    // Hold the strong reference onto the IO bus.
+    _io_bus: Arc<devices::Bus>,
 }
 
 impl Vm {
@@ -329,6 +331,8 @@ impl Vm {
             .ok_or(Error::CreateSystemAllocator)?,
         ));
 
+        let io_bus = Arc::new(devices::Bus::new());
+
         let memory_config = config.lock().unwrap().memory.clone();
 
         let memory_manager = MemoryManager::new(
@@ -351,6 +355,7 @@ impl Vm {
             &exit_evt,
             &reset_evt,
             vmm_path,
+            &io_bus,
         )
         .map_err(Error::DeviceManager)?;
 
@@ -379,6 +384,7 @@ impl Vm {
             state: RwLock::new(VmState::Created),
             cpu_manager,
             memory_manager,
+            _io_bus: io_bus,
         })
     }