code: def _look_for_interface(self, network_backend):
          
  
          result = yield from self._execute("showvminfo", [self._vmname, "--machinereadable"])
          interface = -1
          for info in result.splitlines():
              if '=' in info:
                  name, value = info.split('=', 1)
                  if name.startswith("nic") and value.strip('"') == network_backend:
                      try:
                          interface = int(name[3:])
                          break
                      except ValueError:
                          continue
          return interface
docs: Look for an interface with a specific network backend.
  
          :returns: interface number or -1 if none is found