python/qemu: Add ConsoleSocket for optional use in QEMUMachine

python/qemu: Add ConsoleSocket for optional use in QEMUMachine

We add the ConsoleSocket object, which has a socket interface
and which will consume all arriving characters on the
socket, placing them into an in memory buffer.
This will also provide those chars via recv() as
would a regular socket.
ConsoleSocket also has the option of dumping
the console bytes to a log file.

We also give QEMUMachine the option of using ConsoleSocket
to drain and to use for logging console to a file.
By default QEMUMachine does not use ConsoleSocket.

This is added in preparation for use by basevm.py in a later commit.
This is a workaround we found was needed for basevm.py since
there is a known issue where QEMU will hang waiting
for console characters to be consumed.

Cc: Eduardo Habkost <[email protected]>
Cc: Cleber Rosa <[email protected]>
Signed-off-by: Robert Foley <[email protected]>
Reviewed-by: Peter Puhov <[email protected]>
Acked-by: Alex Bennée <[email protected]>
Tested-by: Philippe Mathieu-Daudé <[email protected]>
Signed-off-by: Alex Bennée <[email protected]>
Message-Id: <20200601211421[email protected]>
Message-Id: <20200701135652[email protected]>