piix: gigabyte alignment for ram

piix: gigabyte alignment for ram

Map 3G (i440fx) of memory below 4G, so the RAM pieces
are nicely aligned to gigabyte borders.

Keep old memory layout for (a) old machine types and (b) in case all
memory fits below 4G and thus we don't have to split RAM into pieces
in the first place.  The later makes sure this change doesn't take
away memory from 32bit guests.

So, with i440fx and up to 3.5 GB of memory, all of it will be mapped
below 4G.  With more than 3.5 GB of memory 3 GB will be mapped below
4G and the remaining amount will be mapped above 4G.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>