VMware: use session.call_method to invoke api's

All api invocations should use session.call_method, which has a
retry mechanism to handle session failures. Few methods in vmops
that are not using this wrapper to invoke api's, fail
intermittently when the session is not authenticated.

This patch fixes those methods. This issue is more pronounced
during VMware CI (Minesweeper), where random tempest tests fail.

The existing tests for vmops and those added for the retrying
session in https://review.openstack.org/#/c/58890/ should cover
this change, as we are re-routing the api invocation.

Closes-Bug: #1260075
Change-Id: I2beb7c92a4c3f53fd2120629e8f72bfd54d13740
Merge "VMware: use session.call_method to invoke api's"