Revert "Start virtualbox VM one by one" because it doesn't fix the issue

This reverts commit da72a9501a.

gns3server/modules/virtualbox/__init__.py

@@ -41,9 +41,6 @@ class VirtualBox(BaseManager):
 
         super().__init__()
         self._vboxmanage_path = None
-        # It seem starting two VM in paralell can be an issue:
-        # https://github.com/GNS3/gns3-server/issues/190
-        self.start_lock = asyncio.Lock()
         self._execute_lock = asyncio.Lock()
 
     @property

gns3server/modules/virtualbox/virtualbox_vm.py

@@ -178,7 +178,6 @@ class VirtualBoxVM(BaseVM):
         if vm_state != "poweroff" and vm_state != "saved":
             raise VirtualBoxError("VirtualBox VM not powered off or saved")
 
-        with (yield from self.manager.start_lock):
         yield from self._set_network_options()
         yield from self._set_serial_console()