qemu: domain: Clarify temp variable scope in qemuDomainDetermineDiskChain

qemu: domain: Clarify temp variable scope in qemuDomainDetermineDiskChain

The function at first validates the top image of the chain, then
traverses the chain as declared in the XML (if any) and then procedes to
detect the rest of the chain from images. All of the steps have their
own temporary iterator.

Clarify the use scope of the steps by introducing a new temp variable
holding the top level source and adding comments.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: John Ferlan <jferlan@redhat.com>