pb: Introduce pb message types and cr-descriptors

The protobuf-c generate plain routines for entries manipulations, but
we want to have some "generic" way of working with messages. Collect them
all in an array of descriptors (similar to image files descriptions) and
do full typechecking while this.

Such thing will allow to simplfy code later.

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>