Fix use of extract_flavor() in hyper-v driver

This converts a use of extract_flavor() in the hyper-v driver to
instance.get_flavor().

Related to blueprint flavor-from-sysmeta-to-blob

Change-Id: I91d568e6eb97965bd4f406d0c0f047011ec50b78
Merge "Fix use of extract_flavor() in hyper-v driver"