memory: add memory_region_flush_rom_device()

memory: add memory_region_flush_rom_device()

ROM devices go via MemoryRegionOps->write() callbacks for write
operations and do not dirty/invalidate that memory.  Device emulation
must be able to mark memory ranges that have been modified internally
(e.g. using memory_region_get_ram_ptr()).

Introduce the memory_region_flush_rom_device() API for this purpose.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-id: 20190123212234.32068-2-stefanha@redhat.com
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
[PMM: fix block comment style]
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>