vga: adjust dirty memory region if pel panning is active

vga: adjust dirty memory region if pel panning is active

When pel panning is active, one more byte is read from each of the VGA
memory planes.  This has to be accounted in the computation of region_end,
otherwise vga_draw_graphic() fails an assertion:

qemu-system-i386: ../system/physmem.c:946: cpu_physical_memory_snapshot_get_dirty: Assertion `start + length <= snap->end' failed.

Reported-by: Helge Konetzka <hk@zapateado.de>
Resolves: https://gitlab.com/qemu-project/qemu/-/issues/2244
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>