When we need to detect a chain for a image which will become the new
source for a disk (e.g. after a disk media change or a blockjob) we'd
need to replace disk->src temporarily to do so.

Move the 'disksrc' temporary variable to an argument and adjust callers.
Signed-off-by: NPeter Krempa <pkrempa@redhat.com>
Reviewed-by: NJohn Ferlan <jferlan@redhat.com>