Fix crash when virtualbox list of VMS return an empty line

Fix #206

gns3server/modules/virtualbox/__init__.py

@@ -168,7 +168,7 @@ class VirtualBox(BaseManager):
         vms = []
         result = yield from self.execute("list", ["vms"])
         for line in result:
-            if line[0] != '"' or line[-1:] != "}":
+            if len(line) == 0 or line[0] != '"' or line[-1:] != "}":
                 continue # Broken output (perhaps a carriage return in VM name
             vmname, _ = line.rsplit(' ', 1)
             vmname = vmname.strip('"')