qtest: add qtest_server_send abstraction

qtest_server_send is a function pointer specifying the handler used to
transmit data to the qtest client. In the standard configuration, this
calls the CharBackend handler, but now it is possible for other types of
handlers, e.g direct-function calls if the qtest client and server
exist within the same process (inproc)

Signed-off-by: Alexander Bulekov <alxndr@bu.edu>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Darren Kenny <darren.kenny@oracle.com>
Acked-by: Thomas Huth <thuth@redhat.com>
Message-id: 20200220041118.23264-6-alxndr@bu.edu
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>