Add `virtual_size` to Glance's API v2

This patch adds the knowledge of a virtual_size field to Glance's API
v2. The virtual_size field should respect the same rules applied to the
size field in terms of readability, access control and propagation.

Glance's API v1 has been left unmodified.

docImpact
Implements blueprint: split-image-size

Change-Id: Ie4f58ee2e4da3a6c1229840295c7f62023a95b70