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>

python/libvirt-override-api.xml

@@ -277,5 +277,12 @@
       <arg name='flags' type='unsigned int' info='flags, curently unused'/>
       <return type='str *' info='the list of Names of None in case of error'/>
     </function>
+    <function name='virDomainRevertToSnapshot' file='python'>
+      <info>revert the domain to the given snapshot</info>
+      <arg name='dom' type='virDomainPtr' info='dummy domain pointer'/>
+      <arg name='snap' type='virDomainSnapshotPtr' info='pointer to the snapshot'/>
+      <arg name='flags' type='unsigned int' info='flags, curently unused'/>
+      <return type='int' info="0 on success, -1 on error"/>
+    </function>
   </symbols>
 </api>