CA-356645: use "self.session is None" not "self.session == None"
CA-356645: use "self.session is None" not "self.session == None"

Python runtime converts use of == to a call to "__eq__" on the object
and this is not implemented by the Xapi session XMLRPC proxy. After
https://github.com/xapi-project/xen-api/pull/4464 this code causes an
exception.

Signed-off-by: Mark Syms <655512b2a8f1aba6e4085462b623575da824f038@citrix.com>

xapi-project/sm,xapi-project/sm,xapi-project/sm