acpi-build: tweak acpi migration limits

- Tweak error message for legacy machine type:
  Basically if table size exceeds the limits we set all
  bets are off for migration: e.g. it can start failing even
  within given qemu minor version simply because of a bugfix.
- Increase table size to 128k.
- Make sure we notice it long before we start getting close to the
  128k limit: warn at 64k.
- Don't fail if we exceed the limit: most people don't care about
  migration, even less people care about cross version miration.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
