Add helper library for testing the qemu monitor code

Add helper library for testing the qemu monitor code

To be able to test the QEMU monitor code, we need to have a fake
QEMU monitor server. This introduces a simple (dumb) framework
that can do this. The test case registers a series of items to
be sent back as replies to commands that will be executed. A
thread runs the event loop looking for incoming replies and
sending back this pre-registered data. This allows testing all
QEMU monitor code that deals with parsing responses and errors
from QEMU, without needing QEMU around

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>