trace: avoid SystemTap "char const" warnings

trace: avoid SystemTap "char const" warnings

SystemTap's dtrace(1) produces the following warning when it encounters
"char const" instead of "const char":

  Warning: /usr/bin/dtrace:trace-dtrace-root.dtrace:66: syntax error near:
  probe flatview_destroy_rcu

  Warning: Proceeding as if --no-pyparsing was given.

This is a limitation in current SystemTap releases.  I have sent a patch
upstream to accept "char const" since it is valid C:

  https://sourceware.org/ml/systemtap/2018-q1/msg00017.html

In QEMU we still wish to avoid warnings in the current SystemTap
release.  It's simple enough to replace "char const" with "const char".

I'm not changing the documentation or implementing checks to prevent
this from occurring again in the future.  The next release of SystemTap
will hopefully resolve this issue.

Cc: Daniel P. Berrange <berrange@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Daniel P. Berrange <berrange@redhat.com>
Message-id: 20180201162625.4276-1-stefanha@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>