VMware: do not cache image when root_gb is 0

Save space on datastore when the instance size has root_gb of
0. There is no need to cache the disk as it already exists on
the datastore.

TrivialFix

Change-Id: Iac75d97f40a1a91b8fa2df7fb4318fc2965fd93a
Closes-bug: #1304593
Merge "VMware: do not cache image when root_gb is 0"