VMware:Support for attaching disk to backing

VMware:Support for attaching disk to backing

This change adds support for attaching a virtual disk to an existing
backing VM. Currently volume creation from preallocated/sparse 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 requires the above mentioned
method. This change also sets the disk size of backing to at least 1MB which
is the minimum required by VIM APIs.

Closes-Bug: #1340315
Partial-Bug: #1284284
Partial-Bug: #1287185
Partial-Bug: #1287176

Change-Id: Icdb1f26d5c8eaa42408a14c57c7394d96dda078f