Add serialize_object() helper function

This function converts a SUDS object to a dictionary.

Originally implemented in Nova as "object_to_dict()", this function
moves to oslo.vmware to stop leaking the abstraction around the backing
SOAP library into depending projects. This is especially necessary since
we want to switch the backing SOAP library in [1].

[1] https://specs.openstack.org/openstack/oslo-specs/specs/victoria/oslo-vmware-soap-library-switch.html

Change-Id: Ie1d42609104e604f9386c1b1a46be7dcd286e855