macio: Put "macio-nvram" device on the macio bus

macio: Put "macio-nvram" device on the macio bus

macio_oldworld_init() creates a "macio-nvram", sysbus device, but
neglects to but it on a bus.

Put it on the macio bus.  Affects machine g3beige.  Visible in "info
qtree":

             bus: macio.0
               type macio-bus
               [...]
    +          dev: macio-nvram, id ""
    +            size = 8192 (0x2000)
    +            it_shift = 4 (0x4)

This also makes it a QOM child of macio-oldworld.  Visible in "info
qom-tree":

     /machine (g3beige-machine)
       [...]
       /unattached (container)
         [...]
         /device[6] (macio-oldworld)
           [...]
    -    /device[7] (macio-nvram)
    -      /macio-nvram[0] (qemu:memory-region)
    +      /nvram (macio-nvram)
    +        /macio-nvram[0] (qemu:memory-region)
         [rest of device[*] renumbered...]

Cc: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Cc: David Gibson <david@gibson.dropbear.id.au>
Cc: qemu-ppc@nongnu.org
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20200609122339.937862-15-armbru@redhat.com>