migration: Unfold control_save_page()

migration: Unfold control_save_page()

control_save_page() is for RDMA only, unfold it to make the code more
clear.
In addition:
 - Similar to other branches style in ram_save_target_page(), involve RDMA
   only if the condition 'migrate_rdma()' is true.
 - Further simplify the code by removing the RAM_SAVE_CONTROL_NOT_SUPP.

Reviewed-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Li Zhijian <lizhijian@fujitsu.com>
Message-ID: <20250305062825.772629-6-lizhijian@fujitsu.com>
Signed-off-by: Fabiano Rosas <farosas@suse.de>