XenAPI: Fix ValueError in test_slave_asks_master_to_add_slave_to_pool
XenAPI: Fix ValueError in test_slave_asks_master_to_add_slave_to_pool

Change If9e8dd5cc2634168910d5f9f8d9302aeefa16097 in
oslo.serialization will raise a ValueError when it can't
convert something to a primitive, which in this case is the
Aggregate object used in this test. Given the add_to_aggregate
method doesn't even serialize the Aggregate object passed in,
I'm not sure how this was working before, but the fix is to
just not try and expect add_aggregate_host RPC API is called
with a serialized Aggregate object.

Change-Id: I139533dfd94a1aa7a00a6220a4411bebe1c9e88b
Related-Bug: #1593641

Juniper/nova,jianghuaw/nova,vmturbo/nova,rahulunair/nova,phenoxim/nova,jianghuaw/nova,rahulunair/nova,mikalstill/nova,mikalstill/nova,mahak/nova,klmitch/nova,mahak/nova,jianghuaw/nova,phenoxim/nova,gooddata/openstack-nova,rahulunair/nova,gooddata/openstack-nova,vmturbo/nova,gooddata/openstack-nova,openstack/nova,vmturbo/nova,klmitch/nova,klmitch/nova,mahak/nova,vmturbo/nova,openstack/nova,Juniper/nova,klmitch/nova,openstack/nova,Juniper/nova,mikalstill/nova,jianghuaw/nova,Juniper/nova,gooddata/openstack-nova