memory: Add internal memory_region_register_ram function

memory: Add internal memory_region_register_ram function

Factor out common operation from memory_region_init_{ram,rom}
functions to register the region for migration. This avoids
duplicating the long comment in several functions.

Signed-off-by: BALATON Zoltan <balaton@eik.bme.hu>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Akihiko Odaki <odaki@rsg.ci.i.u-tokyo.ac.jp>
Link: https://lore.kernel.org/r/1f0bcb38e59456aa910376df6a375d63c2f3f442.1772924151.git.balaton@eik.bme.hu
Signed-off-by: Peter Xu <peterx@redhat.com>