trace: switch position of headers to what Meson requires

Meson doesn't enjoy the same flexibility we have with Make in choosing
the include path.  In particular the tracing headers are using
$(build_root)/$(<D).

In order to keep the include directives unchanged,
the simplest solution is to generate headers with patterns like
"trace/trace-audio.h" and place forwarding headers in the source tree
such that for example "audio/trace.h" includes "trace/trace-audio.h".

This patch is too ugly to be applied to the Makefiles now.  It's only
a way to separate the changes to the tracing header files from the
Meson rewrite of the tracing logic.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

scripts/tracetool/format/tcg_helper_c.py

@@ -41,7 +41,7 @@ def vcpu_transform_args(args, mode):
 
 def generate(events, backend, group):
     if group == "root":
-        header = "trace-root.h"
+        header = "trace/trace-root.h"
     else:
         header = "trace.h"