Fix crash if VirtualBox doesn't return API version

Fix #136

gns3server/modules/virtualbox/virtualbox_vm.py

@@ -139,6 +139,8 @@ class VirtualBoxVM(BaseVM):
     def create(self):
 
         yield from self._get_system_properties()
+        if "API version" not in self._system_properties:
+            raise VirtualBoxError("Can't access to VirtualBox API Version")
         if parse_version(self._system_properties["API version"]) < parse_version("4_3"):
             raise VirtualBoxError("The VirtualBox API version is lower than 4.3")
         log.info("VirtualBox VM '{name}' [{id}] created".format(name=self.name, id=self.id))