axfer: add support for libffado transmission backend

axfer: add support for libffado transmission backend

At present, axfer is designed to use several types of backend for
transmission of data frames. This commit is an implementation
example of the backend.

Libffado is a userspace library for transmission of data frames according
to protocols similar to IEC 61883-1/6. This library handles audio and
music units on IEEE 1394 bus.

Unfortunately, this library executes ctor/dtor of instances for some
objects in startup/finish routines of C runtime. As a result, it outputs
some superfluous messages even if the backend is not actually used.
Furthermore, this library brings memory leak internally. Therefore,
it's not practical to build this backend for generic purposes. Although
the backend implementation works fine, this commit is just for technical
preview.

Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Signed-off-by: Takashi Iwai <tiwai@suse.de>