VMware: enable booting an ISO with root disk size 0

There may be cases where booting from an ISO does not require
any local disk. In this case the flavor may have the root disk
size set at 0.

Change-Id: Id290a0ede11c754f686d3b70322c84c03f161637
Closes-bug: #1292633
Merge "VMware: enable booting an ISO with root disk size 0"