vga: fix display update region calculation

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: fec5e8c9Reported-by: NKevin Wolf <kwolf@redhat.com>
Reported-by: N李强 <liqiang6-s@360.cn>
Signed-off-by: NGerd Hoffmann <kraxel@redhat.com>
Message-id: 20170509104839.19415-1-kraxel@redhat.com

hw/display/vga.c

@@ -1630,7 +1630,7 @@ static void vga_draw_graphic(VGACommonState *s, int full_update)
     if (!full_update) {
         vga_sync_dirty_bitmap(s);
         snap = memory_region_snapshot_and_clear_dirty(&s->vram, addr1,
-                                                      bwidth * height,
+                                                      line_offset * height,
                                                       DIRTY_MEMORY_VGA);
     }