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 <stefanha@redhat.com>
Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Message-id: 20170125161417.31949-7-berrange@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
diff --git a/qom/cpu.c b/qom/cpu.c
index e815db7..d57faf3 100644
--- a/qom/cpu.c
+++ b/qom/cpu.c
@@ -29,7 +29,7 @@
 #include "qemu/error-report.h"
 #include "sysemu/sysemu.h"
 #include "hw/qdev-properties.h"
-#include "trace.h"
+#include "trace-root.h"
 
 bool cpu_exists(int64_t id)
 {