libqtest: add a layer of abstraction to send/recv

libqtest: add a layer of abstraction to send/recv

This makes it simple to swap the transport functions for qtest commands
to and from the qtest client. For example, now it is possible to
directly pass qtest commands to a server handler that exists within the
same process, without the standard way of writing to a file descriptor.

Signed-off-by: Alexander Bulekov <alxndr@bu.edu>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Darren Kenny <darren.kenny@oracle.com>
Message-id: 20200220041118.23264-7-alxndr@bu.edu
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>