scripts/qemu.py: introduce set_console() method

scripts/qemu.py: introduce set_console() method

The set_console() method is intended to ease higher level use cases
that require a console device.

The amount of intelligence is limited on purpose, requiring either the
device type explicitly, or the existence of a machine (pattern)
definition.

Because of the console device type selection criteria (by machine
type), users should also be able to define that.  It'll then be used
for both '-machine' and for the console device type selection.

Users of the set_console() method will certainly be interested in
accessing the console device, and for that a console_socket property
has been added.

Signed-off-by: Cleber Rosa <crosa@redhat.com>
Message-Id: <20180530184156.15634-5-crosa@redhat.com>
Tested-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>