tests: expand coverage of socket chardev test

tests: expand coverage of socket chardev test

The current socket chardev tests try to exercise the chardev socket
driver in both server and client mode at the same time. The chardev API
is not very well designed to handle both ends of the connection being in
the same process so this approach makes the test case quite unpleasant
to deal with.

This splits the tests into distinct cases, one to test server socket
chardevs and one to test client socket chardevs. In each case the peer
is run in a background thread using the simpler QIOChannelSocket APIs.
The main test case code can now be written in a way that mirrors the
typical usage from within QEMU.

In doing this recfactoring it is possible to greatly expand the test
coverage for the socket chardevs to test all combinations except for a
server operating in blocking wait mode.

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
Message-Id: <20190211182442.8542-16-berrange@redhat.com>
Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>