Don't attempt to read from a stream if it is closed

Don't attempt to read from a stream if it is closed

The I/O event callback processes incoming packets first, and then
does outgoing packets. If the incoming packet caused the stream to
close, then the attempt to process outgoing data resulted in an
error. This caused libvirt to then send an error back to the client,
but the stream had already been stopped. This confused the client
since it sees 2 error events.

* daemon/stream.c: Don't attempt read if stream is closed