VMware: ensure that datastore name exists prior to deleting disk

Treats cases when there is no datastore defined on the VM. This
can result from a exception in the spawn method prior to setting
the datastore for the VM.

Closes-bug: #1276644

Change-Id: I808e1cdaace344da021853d0ef224cadee5a8e05
Merge "VMware: ensure that datastore name exists prior to deleting disk"