VMware: allow selection of vSAN datastores

With support for handling of stream-optimized disk images added, the
vmware driver is now able to deploy to vSAN datastores, so add support
for the the 'vsan' datastore type, and update the datastore selection
logic to take this type into account when when spawning a
stream-optimized image.

Implements: blueprint vmware-vsan-support
Co-authored-by: Radoslav Gerganov <rgerganov@vmware.com>

Change-Id: Id43ad4adcad212a8e0d8ab3830af62d4afda1442
Merge "VMware: allow selection of vSAN datastores"