Look for an interface with a specific network backend.

        :returns: interface number or -1 if none is found

def _look_for_interface(self, network_backend):
        """
        Look for an interface with a specific network backend.

        :returns: interface number or -1 if none is found
        """

        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