migration/channel: Centralize calling migration_channel_connect_outgoing

migration/channel: Centralize calling migration_channel_connect_outgoing

Make the synchronous calls evident by not hiding the call to
migration_channel_connect_outgoing() in the transport code. Have those
functions return and call the function at the upper level.

This helps with navigation: the transport code returns the ioc,
there's no need to look into them when browsing the code.

It also allows RDMA in the source side to use the same path as the
rest of the transports.

While here, document the async calls which are the exception.

Reviewed-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Prasad Pandit <pjp@fedoraproject.org>
Link: https://lore.kernel.org/qemu-devel/20260123141656.6765-26-farosas@suse.de
Signed-off-by: Fabiano Rosas <farosas@suse.de>