Fix memory dirty getting API change fallout

Fix confusion in length calculation in commit
cd7a45c95ecf2404810f3c6becb7cb83c5010ad8.

Reported-by: Jan Kiszka <jan.kiszka@web.de>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>
diff --git a/hw/framebuffer.c b/hw/framebuffer.c
index ea122fb..f4747cd 100644
--- a/hw/framebuffer.c
+++ b/hw/framebuffer.c
@@ -87,7 +87,7 @@
     dest += i * dest_row_pitch;
 
     for (; i < rows; i++) {
-        dirty = memory_region_get_dirty(mem, addr, addr + src_width,
+        dirty = memory_region_get_dirty(mem, addr, src_width,
                                              DIRTY_MEMORY_VGA);
         if (dirty || invalidate) {
             fn(opaque, dest, src, cols, dest_col_pitch);