virDomainGetJobStats destroys the completed statistics on the first
read. Give the user possibility to keep them around if they wish so.

Add a flag VIR_DOMAIN_JOB_STATS_KEEP_COMPLETED which will read the stats
without destroying them.
Signed-off-by: NPeter Krempa <pkrempa@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>