memory: Align MemoryRegionSections fields

MemoryRegionSection includes an Int128 'size' field;
on some platforms the compiler causes an alignment of this to
a 128bit boundary, leaving 8 bytes of dead space.
This deadspace can be filled with junk.

Move the size field to the top avoiding unnecessary alignment.

Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20190814175535.2023-2-dgilbert@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>