Since blockcommit is asynchronous, libvirtd can be restarted while the
operation runs.  To ensure the information necessary to finish up the job
is not lost, serialisation to and deserialisation from the status XML is
added.

To unittest this, the new element was only added to the active commit test,
the non-active commit test doesn't have the new element so as to test its
absence.
Signed-off-by: NPavel Mores <pmores@redhat.com>
Reviewed-by: NPeter Krempa <pkrempa@redhat.com>