trace: switch to modular code generation for sub-directories

trace: switch to modular code generation for sub-directories

Introduce rules in the top level Makefile that are able to generate
trace.[ch] files in every subdirectory which has a trace-events file.

The top level directory is handled specially, so instead of creating
trace.h, it creates trace-root.h. This allows sub-directories to
include the top level trace-root.h file, without ambiguity wrt to
the trace.g file in the current sub-dir.

Reviewed-by: Stefan Hajnoczi <[email protected]>
Signed-off-by: Daniel P. Berrange <[email protected]>
Message-id: 20170125161417[email protected]
Signed-off-by: Stefan Hajnoczi <[email protected]>