network: Don't use ERR_NO_SUPPORT for invalid net-update requests

network: Don't use ERR_NO_SUPPORT for invalid net-update requests

VIR_ERR_NO_SUPPORT maps to the error string

    this function is not supported by the connection driver

and is largely only used for when a driver doesn't have any
implementation for a public API. So its usage with invalid
net-update requests is a bit out of place. Instead use
VIR_ERR_OPERATION_UNSUPPORTED which maps to:

    Operation not supported

And is what qemu's hotplug routines use in similar scenarios