qemu: blockjob: Implement concluded blockjob handler for backup blockjobs

qemu: blockjob: Implement concluded blockjob handler for backup blockjobs

After the individual sub-blockjobs of a backup libvirt job finish we
must detect it and notify the parent job, so that it can be properly
terminated.

Since we update job information to determine success of a blockjob we
can directly report back also statistics of the blockjob.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>