trace: disallow more than 10 arguments per trace event

The UST trace backend can only cope with upto 10 arguments. To ensure we
don't exceed the limit when UST is not compiled in, disallow more than
10 arguments upfront.

This prevents the case where:

  commit 0fc8aec7
  Author: Zhang Chen <zhangchen.fnst@cn.fujitsu.com>
  Date:   Tue Apr 18 10:20:20 2017 +0800

    COLO-compare: Optimize tcp compare trace event

    Optimize two trace events as one, adjust print format make
    it easy to read. rename trace_colo_compare_pkt_info_src/dst
    to trace_colo_compare_tcp_info.

regressed the fix done in

  commit 2dfe5113
  Author: Alex Bennée <alex.bennee@linaro.org>
  Date:   Fri Oct 28 14:25:59 2016 +0100

    net: split colo_compare_pkt_info into two trace events

    It seems there is a limit to the number of arguments a UST trace event
    can take and at 11 the previous trace command broke the build. Split the
    trace into a src pkt and dst pkt trace to fix this.
Signed-off-by: NAlex Bennée <alex.bennee@linaro.org>
    Message-id: 20161028132559.8324-1-alex.bennee@linaro.org
Reviewed-by: NPeter Maydell <peter.maydell@linaro.org>
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>

Now we get an immediate fail even when UST is disabled:

  GEN     net/trace.h
Traceback (most recent call last):
  File "/home/berrange/src/virt/qemu/scripts/tracetool.py", line 154, in <module>
    main(sys.argv)
  File "/home/berrange/src/virt/qemu/scripts/tracetool.py", line 145, in main
    events.extend(tracetool.read_events(fh))
  File "/home/berrange/src/virt/qemu/scripts/tracetool/__init__.py", line 307, in read_events
    event = Event.build(line)
  File "/home/berrange/src/virt/qemu/scripts/tracetool/__init__.py", line 244, in build
    event = Event(name, props, fmt, args)
  File "/home/berrange/src/virt/qemu/scripts/tracetool/__init__.py", line 196, in __init__
    "argument count" % name)
ValueError: Event 'colo_compare_tcp_info' has more than maximum permitted argument count
Makefile:96: recipe for target 'net/trace.h-timestamp' failed
Signed-off-by: NDaniel P. Berrange <berrange@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
Message-id: 20170426153900.21066-1-berrange@redhat.com
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>

scripts/tracetool/__init__.py

@@ -191,6 +191,10 @@ class Event(object):
         self.event_trans = event_trans
         self.event_exec = event_exec
 
+        if len(args) > 10:
+            raise ValueError("Event '%s' has more than maximum permitted "
+                             "argument count" % name)
+
         if orig is None:
             self.original = weakref.ref(self)
         else: