VMware:Support for create disk and disk descriptor

This change adds utility methods for creating a virtual disk and virtual
disk descriptor. Currently volume creation from preallocated image doesn't
honour the disk type in the volume extra spec and adapter type in the image
meta-data. The workflow to address these issues require support for the
above mentioned methods.

Change-Id: If8ec1fd6c60d26552e5fb58b8bb16a8cc87d9d28
Partial-Bug: #1284284
Partial-Bug: #1287185
Merge "VMware:Support for create disk and disk descriptor"