VMware: factor out management of controller keys and unit numbers

This patch encapsulates the allocation of controller key and unit
number for a new virtual disk/CDROM into utility function.
This way high-level methods like spawn, rescue, etc. don't have
to deal with implementation details like the above and the
algorithm for allocating controller_key and unit_number is
clearly defined and can be easily reused.

Change-Id: Ia6d5db5d5c5fa25f680eae82892dcb9b2be864fa
Closes-Bug: #1259981
Merge "VMware: factor out management of controller keys and unit numbers"