VMware: Use datastore_regex for disk stats

VMware driver does not use datastore_regex when reporting disk
stats, while it is used when spawning instances.  As a result,
the available disk_free reported periodically can be different
from what is available when spawning.

Change-Id: Ibeb69afc5183c22191b77d45c73dcc8b0ef6d1f5
Closes-Bug: #1309753
Co-Authored-By: Sabari Kumar Murugesan <smurugesan@vmware.com>
Co-Authored-By: Davanum Srinivas <dims@linux.vnet.ibm.com>