First pass at adding ability for Memcheck to print all output in XML

First pass at adding ability for Memcheck to print all output in XML
form.  The relevant flag is --xml=yes.  Currently this only works with
Memcheck.

Specifying this flag fixes various other options relating to verbosity
and behaviour of the leak checker, so that the resulting output is in
a relatively fixed form suitable for parsing by GUIs.

Still to do:

* Add mechanism to show error counts
* Add regression test
* Document the resulting format

git-svn-id: svn://svn.valgrind.org/valgrind/trunk@3773