VMware:Disk type conversion during clone backing

This patch adds support for converting virtual disk provisioning
type during clone backing operation. Currently volume creation from
preallocated/sparse image doesn't honour the disk type property in
the volume extra spec. The workflow to address this issue requires
disk provisioning type conversion during clone.

Partial-Bug: #1287176
Partial-Bug: #1287185

Change-Id: Ie493768445b838d3e9447edb76d9845e972bc7c3
Merge "VMware:Disk type conversion during clone backing"