Use real disk size to consider a resize down

When resize an instance, we don't allow a resize down operation.
Currently, a flavor ephemeral size will be used to do the check,
but usually we might create ephemeral size less than flavor
ephemeral size. This patch use the real disk size to do the resize
instead of the flavor ephemeral size.

Also, the test cases are updated to change from dict to objects,
if we change the flavor from dict to objects will benefit from it.

Change-Id: I03006a2ee8edf310a3ad374ebb7e4cd4034f8250
Closes-Bug: #1324005
Merge "Use real disk size to consider a resize down"