Recommend vCenter VMDK driver and other edits

Updated the VMware VMDK driver doc to recommend the vCenter VMDK
driver since the ESX VMDK driver has not been tested extensively.
Also made other edits to make the document easier to read.

backport: havana

Fixes bug #1246913

Change-Id: Iae910c59d0950cb19d18cded6a99842a1cbfd8b6
(cherry picked from commit 87d8f50d89ec2256cac5d0a270d8457bb98547e4)