macio: set user_creatable to false in macio_class_init()

macio: set user_creatable to false in macio_class_init()

Commit 348b8d1a76 "macio: don't reference serial_hd() directly within the device"
removed the setting of user_creatable to false on the basis that the restriction
was due to the use of serial_hd() in macio_instance_init().

Unfortunately this isn't the full story since the PIC object property links
must still be set before the device is realized. Whilst it is possible to update
the macio device and Mac machines to resolve this, the fix is too invasive at
this point in the release cycle.

For now simply set user_creatable back to false in macio_class_init() to
prevent QEMU from segfaulting in anticipation of the proper fix arriving in
QEMU 6.0.

Reported-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20201110103111.18395-1-mark.cave-ayland@ilande.co.uk>
Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>