core: RetrieveImageData - validate received response

After recieving OK response validate that the received size specified in
the header and the size of the data match the requested size.

Change-Id: I8f27ad8349ad9c8f1e44a204a4c5bc31f9ae5c7a
Signed-off-by: Liron Aravot <laravot@redhat.com>