qemu: block-commit: Mark disk in block jobs only on successful command

Patch 51f9f03a introduces a regression
where if a blockCommit operation fails the disk is still marked as being
part of a block job but can't be unmarked later.

src/qemu/qemu_driver.c

@@ -17094,7 +17094,8 @@ qemuDomainBlockCommit(virDomainPtr dom,
         goto endjob;
     }
 
-    disk->blockjob = true;
+    if (ret == 0)
+        disk->blockjob = true;
 
     if (mirror) {
         if (ret == 0) {