python/qemu: Change ConsoleSocket to optionally drain socket.

python/qemu: Change ConsoleSocket to optionally drain socket.

The primary purpose of this change is to clean up
machine.py's console_socket property to return a single type,
a ConsoleSocket.

ConsoleSocket now derives from a socket, which means that
in the default case (of not draining), machine.py
will see the same behavior as it did prior to ConsoleSocket.

Signed-off-by: Robert Foley <robert.foley@linaro.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20200717203041.9867-3-robert.foley@linaro.org>
Message-Id: <20200724064509.331-16-alex.bennee@linaro.org>