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
Merge "Recommend vCenter VMDK driver and other edits"