fix leak in memoryStats with older python

fix leak in memoryStats with older python

libvirt_virDomainMemoryStats() function creates a dictionary without
any checks whether the additions were successful, whether the python
objects were created and, most importantly, without decrementing the
reference count on the objects added to the dictionary.  This is
somehow not an issue with current upstream versions, however with
python 2.6 this exposes a leak in our bindings.  The following patch
works on both old and new CPython versions and is already used in
other parts of the code, so it's also most straightforward.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1099860

Signed-off-by: Martin Kletzander <mkletzan@redhat.com>