Allow virConnect to be used as a context manager

The libvirt python bindings are now more 'pythonic' as virConnect can
now be used as a context manager.

For example, it's possible to write the following code:

with libvirt.open() as conn:
    # do something with the connection...
    print(conn.listAllDomains())

At the end of this with-block the connection will be closed
automatically.

Signed-off-by: Marc Hartmayer <mhartmay@linux.vnet.ibm.com>
Reviewed-by: Bjoern Walk <bwalk@linux.vnet.ibm.com>
Reviewed-by: Boris Fiuczynski <fiuczy@linux.vnet.ibm.com>

libvirt-override-virConnect.py

@@ -11,6 +11,12 @@
             libvirtmod.virConnectClose(self._o)
         self._o = None
 
+    def __enter__(self):
+        return self
+
+    def __exit__(self, exc_type_, exc_value_, traceback_):
+        self.close()
+
     def domainEventDeregister(self, cb):
         """Removes a Domain Event Callback. De-registering for a
            domain callback will disable delivery of this event type """