pb-protocol: Don't allocate in deserialise functions

pb-protocol: Don't allocate in deserialise functions

Curently, the protocol deserialise functions are allocating device and
boot_command structures. This (implicitly) makes them responsible for
initialisation of these structures too.

Rather that making the protocol responsible for initialising the devices
and boot commands, this change gives the deserialise functions an
argument to an already-instanciated structure. This means that the
creation is no longer implied by the deserialise.

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>