Prevent double start of VirtualBox VM and VMware VM

Fix https://github.com/GNS3/gns3-gui/issues/1664

gns3server/compute/virtualbox/virtualbox_vm.py

@@ -206,6 +206,9 @@ class VirtualBoxVM(BaseNode):
         Starts this VirtualBox VM.
         """
 
+        if self.status == "started":
+            return
+
         # resume the VM if it is paused
         vm_state = yield from self._get_vm_state()
         if vm_state == "paused":

gns3server/compute/vmware/vmware_vm.py

@@ -386,6 +386,9 @@ class VMwareVM(BaseNode):
         Starts this VMware VM.
         """
 
+        if self.status == "started":
+            return
+
         if (yield from self.is_running()):
             raise VMwareError("The VM is already running in VMware")