We can use the output of 'query-jobs' to figure out some useful
information about a backup job. That is progress in case of a push job
and scratch file use in case of a pull job.

Add a worker which will total up the data and call it from
qemuDomainGetJobStatsInternal.
Signed-off-by: NPeter Krempa <pkrempa@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
Reviewed-by: NJán Tomko <jtomko@redhat.com>