Set root disk's size if it is a local image BDM

While preparing the block device mapping for creating new instance(s),
before sending the request to the manager, try to guess the size of the
root BDM if it is not set. The disk size will be taken either from the
base options (inherited from the instance type) or, if it is not present,
from the image size.

Change-Id: I05144bbf0cef8d69a6ca61b7d95561822a5cf886