qemu: blockjob: Refuse to register blockjob if disk already has one

Most code paths prevent starting a blockjob if we already have one but
the job registering function does not do this check. While this isn't a
problem for regular cases we had a bad test case where we registered two
jobs for a single disk which leaked one of the jobs. Prevent this in the
registering function until we allow having multiple jobs per disk.
Signed-off-by: NPeter Krempa <pkrempa@redhat.com>
ACKed-by: NEric Blake <eblake@redhat.com>
Reviewed-by: NMartin Kletzander <mkletzan@redhat.com>

src/qemu/qemu_blockjob.c

@@ -143,6 +143,12 @@ qemuBlockJobRegister(qemuBlockJobDataPtr job,
 {
     qemuDomainObjPrivatePtr priv = vm->privateData;
 
+    if (disk && QEMU_DOMAIN_DISK_PRIVATE(disk)->blockjob) {
+        virReportError(VIR_ERR_INTERNAL_ERROR,
+                       _("disk '%s' has a blockjob assigned"), disk->dst);
+        return -1;
+    }
+
     if (virHashAddEntry(priv->blockjobs, job->name, virObjectRef(job)) < 0) {
         virObjectUnref(job);
         return -1;