Recheck disk backing chains after snapshot

When a snapshot operation finishes we have to recheck the backing chain
of all disks involved in the snapshot. And we need to do that even if
the operation failed because some of the disks might have changed if
QEMU did not support transactions.

src/qemu/qemu_driver.c

@@ -12949,6 +12949,7 @@ qemuDomainSnapshotCreateDiskActive(virQEMUDriverPtr driver,
     bool persist = false;
     bool reuse = (flags & VIR_DOMAIN_SNAPSHOT_CREATE_REUSE_EXT) != 0;
     virQEMUDriverConfigPtr cfg = virQEMUDriverGetConfig(driver);
+    virErrorPtr orig_err = NULL;
 
     if (!virDomainObjIsActive(vm)) {
         virReportError(VIR_ERR_OPERATION_INVALID,
@@ -13040,6 +13041,17 @@ qemuDomainSnapshotCreateDiskActive(virQEMUDriverPtr driver,
     }
 
  cleanup:
+    /* recheck backing chains of all disks involved in the snapshot */
+    orig_err = virSaveLastError();
+    for (i = 0; i < snap->def->ndisks; i++) {
+        if (snap->def->disks[i].snapshot != VIR_DOMAIN_SNAPSHOT_LOCATION_EXTERNAL)
+            continue;
+        qemuDomainDetermineDiskChain(driver, vm, vm->def->disks[i], true);
+    }
+    if (orig_err) {
+        virSetError(orig_err);
+        virFreeError(orig_err);
+    }
 
     if (ret == 0 || !virQEMUCapsGet(priv->qemuCaps, QEMU_CAPS_TRANSACTION)) {
         if (virDomainSaveStatus(driver->xmlopt, cfg->stateDir, vm) < 0 ||