q35: gigabyte alignment for ram

q35: gigabyte alignment for ram

Map 2G (q35) 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.

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