vga display update mis-calculated the region for the dirty bitmap
snapshot in case the scanlines are padded. This can triggere an
assert in cpu_physical_memory_snapshot_get_dirty().
Fixes: fec5e8c92becad223df9d972770522f64aafdb72
Reported-by: Kevin Wolf <kwolf@redhat.com>
Reported-by: 李强 <liqiang6-s@360.cn>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Message-id: 20170509104839.19415-1-kraxel@redhat.com