VMware: image user functions for spawn()

Refactor each mode of using cached images when configuring a newly
created virtual machine into their own functions.

DocImpact:
vmware_linked_clone image property is ignored when spawning an
image that is an ISO.

partial-blueprint: vmware-spawn-refactor
Change-Id: I863fc0ef644bcf579e74d6f567f14aed98a23c84
Merge "VMware: image user functions for spawn()"