Add a virtual_size attribute to the Image model

In order to distinguish file size from the virtual image size, we need
to add a new field to the Image model to represent that. This patch adds
a new field called `virtual_size` to the Image model and the migration
for it.

The existing size attribute is kept as is in order to keep backwards
compatibility. Most of the users consider that attribute to be the
physical image size, which is what we're trying to achieve here.

The API implementation will be done in a follow-up patch.

Partially-Implements bp: split-image-size

Change-Id: Ie968e8bcfaeda79de003fe8fd7a3b58c3e45f592