VMware: prevent image snapshot if no root disk defined

If there is no root disk defined then the snapshot should
throw an exception.

Change-Id: I0549b8ca9ba41b734277600cb66a74b27ca72d88
Related-bug: #1297605
(cherry picked from commit e0638e29cda39c609ecf518e5adacf70e158d71a)
Merge "VMware: prevent image snapshot if no root disk defined" into stable/icehouse