Fix up the python bindings for snapshotting.

Fix up the python bindings for snapshotting.

This involved a few fixes.  To start with,
an virDomainSnapshot object is really tied to a
domain, not a connection, so we have to generate
a slightly different object so that we can get
at self._dom for the object.

Next, we had to "dummy" up an override piece of
XML with a bogus argument that the function doesn't
actually take.  That's so that the generator places
virDomainRevertToSnapshot underneath the correct
class (namely, the virDomain class).

Finally, we had to hand-implement the
virDomainRevertToSnapshot implementation, ignoring the
bogus pointer we are being passed.

With all of this in place, I was able to successfully
take a snapshot and revert to it using only the
Python bindings.

Signed-off-by: Chris Lalancette <clalance@redhat.com>