Fix rare crash in tests for travis and fix coverage

gns3server/modules/vpcs/__init__.py

@@ -52,7 +52,8 @@ class VPCS(BaseManager):
         vm = self.get_vm(vm_id)
         i = self._used_mac_ids[vm_id]
         self._free_mac_ids[vm.project.id].insert(0, i)
-        del self._used_mac_ids[vm_id]
+        if vm_id in self._used_mac_ids:
+            del self._used_mac_ids[vm_id]
         yield from super().close_vm(vm_id, *args, **kwargs)
         return vm