snapshot: Don't hose list on deletion failure

If qemuDomainSnapshotDiscard() fails for any reason (rare,
but possible with an ill-timed ENOMEM or if
qemuDomainSnapshotForEachQcow2() has problems talking to the
qemu guest monitor), then an attempt to retry the snapshot
deletion API will crash because we didn't undo the effects
of virDomainSnapshotDropParent() temporarily rearranging the
internal list structures, and the second attempt to drop
parents will dereference NULL.  Fix it by instead noting that
there are only two callers to qemuDomainSnapshotDiscard(),
and only one of the two callers wants the parent to be updated;
thus we can move the call to virDomainSnapshotDropParent()
into a code path that only gets executed on success.
Signed-off-by: NEric Blake <eblake@redhat.com>
ACKed-by: NMichal Privoznik <mprivozn@redhat.com>

src/qemu/qemu_domain.c

@@ -8249,7 +8249,7 @@ int
 qemuDomainSnapshotDiscard(virQEMUDriverPtr driver,
                           virDomainObjPtr vm,
                           virDomainSnapshotObjPtr snap,
-                          bool update_current,
+                          bool update_parent,
                           bool metadata_only)
 {
     char *snapFile = NULL;
@@ -8278,7 +8278,7 @@ qemuDomainSnapshotDiscard(virQEMUDriverPtr driver,
         goto cleanup;
 
     if (snap == vm->current_snapshot) {
-        if (update_current && snap->def->parent) {
+        if (update_parent && snap->def->parent) {
             parentsnap = virDomainSnapshotFindByName(vm->snapshots,
                                                      snap->def->parent);
             if (!parentsnap) {
@@ -8301,6 +8301,8 @@ qemuDomainSnapshotDiscard(virQEMUDriverPtr driver,
 
     if (unlink(snapFile) < 0)
         VIR_WARN("Failed to unlink %s", snapFile);
+    if (update_parent)
+        virDomainSnapshotDropParent(snap);
     virDomainSnapshotObjListRemove(vm->snapshots, snap);
 
     ret = 0;

src/qemu/qemu_driver.c

@@ -16526,7 +16526,6 @@ qemuDomainSnapshotDelete(virDomainSnapshotPtr snapshot,
         snap->first_child = NULL;
         ret = 0;
     } else {
-        virDomainSnapshotDropParent(snap);
         ret = qemuDomainSnapshotDiscard(driver, vm, snap, true, metadata_only);
     }