Allow qemu access to modify backing files in case when we want to delete
a checkpoint.

This patch adds tracking of which images need to be relabelled when
calculating the transaction, the code to relabel them and rollback.

To verify that stuff works we also output the list of images to relabel
into the test case output files in qemublocktest.
Signed-off-by: NPeter Krempa <pkrempa@redhat.com>
Reviewed-by: NJán Tomko <jtomko@redhat.com>