Currently the QEMU Python module limits the QEMUMachine class to
use the first serial console.

Some machines/guest might use another console than the first one as
the 'boot console'. For example the Raspberry Pi uses the second
(AUX) console.

To be able to use the Nth console as default, we simply need to
connect all the N - 1 consoles to the null chardev.

Add an index argument, so we can use a specific serial console as
default.

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Liam Merwick <liam.merwick@oracle.com>
Tested-by: Liam Merwick <liam.merwick@oracle.com>
Reviewed-by: Wainer dos Santos Moschetta <wainersm@redhat.com>
Message-Id: <20200120235159.18510-5-f4bug@amsat.org>
[PMD: zero-initialize _console_index in __init__()]
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>