Convert the function to just prepare data for the disk. Callers need to
do the looping since there's more to do than just copy the data around.

The code path in qemuDomainPrepareDiskSource doesn't need to loop over
the chain yet, since there currently is no chain at this point. This
will be addressed later in the blockdev series where we will setup much
more stuff.
Signed-off-by: NPeter Krempa <pkrempa@redhat.com>
Reviewed-by: NJán Tomko <jtomko@redhat.com>