Added context to VMDK driver topic

Added a new section that provides context and background for the
VMDK driver, describing how the VMDK creates a VMDK file, how
that file creation is completed only when it is attached to a
volume.

Change-Id: I6527edab609e6cbaf556a93f5dec4237ab24f1f5
backport: none
Closes-Bug: #1275894
Merge "Added context to VMDK driver topic"