qemu: domain: Refactor control flow in qemuDomainDetermineDiskChain
qemu: domain: Refactor control flow in qemuDomainDetermineDiskChain

Split out clearing of the backing chain prior to other code since it
will be required later and optimize few layers of nested conditions and
loops.

libvirt/libvirt,nertpinx/libvirt,olafhering/libvirt,libvirt/libvirt,jardasgit/libvirt,fabianfreyer/libvirt,fabianfreyer/libvirt,jardasgit/libvirt,jfehlig/libvirt,eskultety/libvirt,jardasgit/libvirt,eskultety/libvirt,crobinso/libvirt,zippy2/libvirt,fabianfreyer/libvirt,andreabolognani/libvirt,eskultety/libvirt,jardasgit/libvirt,jfehlig/libvirt,olafhering/libvirt,zippy2/libvirt,jfehlig/libvirt,zippy2/libvirt,jfehlig/libvirt,andreabolognani/libvirt,nertpinx/libvirt,crobinso/libvirt,andreabolognani/libvirt,nertpinx/libvirt,nertpinx/libvirt,crobinso/libvirt,libvirt/libvirt,eskultety/libvirt,olafhering/libvirt,libvirt/libvirt,jardasgit/libvirt,nertpinx/libvirt,crobinso/libvirt,olafhering/libvirt,andreabolognani/libvirt,fabianfreyer/libvirt,eskultety/libvirt,fabianfreyer/libvirt,zippy2/libvirt,andreabolognani/libvirt