vmm: Enable HTTP response for PCI device hotplug

This patch completes the series by connecting the dots between the HTTP
frontend and the device manager backend.

Any request to hotplug a VFIO, disk, fs, pmem, net, or vsock device will
now return a response including the device name and the place of the
device in the PCI topology.

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

vmm/src/vm.rs

@@ -209,6 +209,9 @@ pub enum Error {
 
     /// No more that one virtio-vsock device
     TooManyVsockDevices,
+
+    /// Failed serializing into JSON
+    SerializeJson(serde_json::Error),
 }
 pub type Result<T> = result::Result<T, Error>;