migration: Move migrate_use_zero_copy_send() to options.c

Once that we are there, we rename the function to
migrate_zero_copy_send() to be consistent with all other capabilities.

We can remove the CONFIG_LINUX guard.  We already check that we can't
setup this capability in migrate_caps_check().

Signed-off-by: Juan Quintela <quintela@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>