migration: Rename ram_compressed_pages() to compress_ram_pages()

We are moving to have all functions exported from ram-compress.c to
start with compress_.

Reviewed-by: Fabiano Rosas <farosas@suse.de>
Signed-off-by: Juan Quintela <quintela@redhat.com>
Message-ID: <20231019110724.15324-12-quintela@redhat.com>