framebuffer: set DIRTY_MEMORY_VGA on RAM that is used for the framebuffer

The MemoryRegionSection contains enough information to access the
RAM region underlying the framebuffer, and can be cached inside the
display device.

By doing this, the new framebuffer_update_memory_section function can
enable dirty memory logging on the relevant RAM region.  The function
must be called whenever the stride or base of the framebuffer changes;
a simple way to cover these cases is to call it on every full frame
invalidation, which is a rare case.

framebuffer_update_display now works entirely on a MemoryRegionSection,
without going through cpu_physical_memory_map/unmap.

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
