Support callbacks on virStream APIs in remote driver client

Support callbacks on virStream APIs in remote driver client

The current remote driver code for streams only supports
blocking I/O mode. This is fine for the usage with migration
but is a problem for more general use cases, in particular
bi-directional streams.

This adds supported for the stream callbacks and non-blocking
I/O. with the minor caveat is that it doesn't actually do
non-blocking I/O for sending stream data, only receiving it.
A future patch will try to do non-blocking sends, but this is
quite tricky to get right.

* src/remote/remote_driver.c: Allow non-blocking I/O for
  streams and support callbacks