core: Introducing RetrieveImageDataVDSCommand

This patch adds RetrieveImageDataVDSCommand which is used to download
image data to the engine up to a given size.
The data is returned by the command as a byte array.

Change-Id: I87be49262b1e06a12f55055360a6c7ddc7486897
Signed-off-by: Liron Aravot <laravot@redhat.com>