python: Implement Pointer ("virDomainPtr", _) (RHBZ#1075164).

This allows the Python binding of guestfs_add_libvirt_dom to work.

There is a regression test to ensure this keeps working.

Note this requires a patched libvirt-python, supporting the
c_pointer() method.

python/run-python-tests

@@ -18,6 +18,9 @@
 
 errors=0
 
+guestsdir="$(cd ../tests/guests && pwd)"
+export guestsdir
+
 for f in $srcdir/t/*.py; do
   $PYTHON $f
   r=$?