qemublocktest: Fix and optimize fake image chain
qemublocktest: Fix and optimize fake image chain

Set the 'id' field of the backing chain properly so that we can look
up images, and initialize 6 images instead of 10 as we don't use more
currently.

Signed-off-by: Peter Krempa <2cf5c04c61aa466e4a47bfedc747d17279c72ffc@redhat.com>
Reviewed-by: Eric Blake <a9da18479c871fe903d9590822cfe2629c6a4d11@redhat.com>

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