qemuBackupDiskDataCleanupOne: Don't exit early when the job has started

qemuBackupDiskDataCleanupOne: Don't exit early when the job has started

Originally the function was cleaning up a failed job only but now
there's other stuff that needs to be cleared too.

Make only steps which clean up after a failed job depend on the
'started' field and execute the rest of the code always.

This fixes a leak of the backup job tracking object and the blockdev-add
helper data.

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