tests/vm: serial console support helpers

tests/vm: serial console support helpers

Add a bunch of helpers to talk to the guest using the
serial console.

Also drop the hard-coded -serial parameter for the vm
so QEMUMachine.set_console() actually works.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Tested-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Tested-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20190617043858.8290-7-kraxel@redhat.com>
[AJB: added tags]
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>