When connecting to a compute node ask for qemu images list

The server start to compute qemu image list in background
because first time discovery could be slow due to the compute
time of md5sum.

Fix #996

gns3server/web/web_server.py

@@ -33,6 +33,7 @@ from .route import Route
 from ..config import Config
 from ..compute import MODULES
 from ..compute.port_manager import PortManager
+from ..compute.qemu import Qemu
 from ..controller import Controller
 
 
@@ -192,6 +193,10 @@ class WebServer:
         Called when the HTTP server start
         """
         yield from Controller.instance().start()
+        # Because with a large image collection
+        # without md5sum already computed we start the
+        # computing with server start
+        asyncio.async(Qemu.instance().list_images())
 
     def run(self):
         """