VMware: fix bug with booting from volumes

When booting from a volume without an image reference would cause
an exception.

Change-Id: Ib0302197b8d4f5e054a8c4426cb0dde7cb82842e
Closes-bug: #1235022
(cherry picked from commit 026843773b71ab0ac30abb36fe9eed92f5e11f7a)
Merge "VMware: fix bug with booting from volumes" into stable/havana