Do not replace the aggregate metadata when updating az

When the aggregate availability zone is updated, the
existing metadata should not be replaced

Change-Id: Ife856af9e7dca836ebe8a1ec050effaad421e17d
Closes-Bug: #1274924
xenapi: add tests for find_bad_volumes

Added some unit tests of the exiting method that appeared to have no
unit test coverage.

Change-Id: I3e2a22c76eb08048032b28a1053f02d93107614c
Merge "xenapi: add tests for find_bad_volumes"
Merge "Do not replace the aggregate metadata when updating az"