memory: Factor out common ram region initialization

memory: Factor out common ram region initialization

Introduce internal helper function to remove duplicated code from
different memory_region_init_*ram functions. Remove local err and
error_propagate and pass errp and check return value instead.
Also shorten some function prototypes while at it.

Signed-off-by: BALATON Zoltan <balaton@eik.bme.hu>
Reviewed-by: Akihiko Odaki <odaki@rsg.ci.i.u-tokyo.ac.jp>
Link: https://lore.kernel.org/r/d6db01c283149b46023ffeb9c5b368c67f6acc8d.1772924151.git.balaton@eik.bme.hu
Signed-off-by: Peter Xu <peterx@redhat.com>