migration: Refactor migration_incoming_setup()

migration: Refactor migration_incoming_setup()

Commit 6720c2b32725 ("migration: check magic value for deciding the
mapping of channels") extracted the only code that could fail in
migration_incoming_setup().

Now migration_incoming_setup() can't fail, so refactor it to return void
and remove errp parameter.

Signed-off-by: Avihai Horon <avihaih@nvidia.com>
Reviewed-by: Fabiano Rosas <farosas@suse.de>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Link: https://lore.kernel.org/r/20231231093016.14204-4-avihaih@nvidia.com
Signed-off-by: Peter Xu <peterx@redhat.com>