Fix invalid vim call in vim_util.get_dynamic_properties()

Code was calling
vim.CancelRetrievePropertiesEx(token=obj_content.token)

This will be translated into a call to
vim_request_handler(managed_object, **kwargs) by Vim.__getattr__(),
which requires a managed_object argument.

This change also adds a test for vim_util.get_dynamic_properties which
exercises both property retrieval, and the cancellation of an
unfinished RetrieveResult.

Change-Id: Ie0d98cbded4dee680593d2a950510de25ee5fab7
Merge "Fix invalid vim call in vim_util.get_dynamic_properties()"