If a backup job fails midway it's hard to figure out what happened as
it's running asynchronous. Use the VIR_DOMAIN_JOB_ERRMSG job statistics
field to pass through the error from the first failed backup-blockjob
so that both the consumer of the virDomainGetJobStats and the
corresponding event can see the error.

event 'job-completed' for domain backup-test:
	operation: 9
	time_elapsed: 46
	disk_total: 104857600
	disk_processed: 10158080
	disk_remaining: 94699520
	success: 0
	errmsg: No space left on device

virsh domjobinfo backup-test --completed --anystats
Job type:         Failed
Operation:        Backup
Time elapsed:     46           ms
File processed:   9.688 MiB
File remaining:   90.312 MiB
File total:       100.000 MiB
Error message:    No space left on device

https://bugzilla.redhat.com/show_bug.cgi?id=1812827Signed-off-by: NPeter Krempa <pkrempa@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>