qemu: forbid snapshot-delete --children-only on external snapshot

https://bugzilla.redhat.com/show_bug.cgi?id=956506 documents that
given a domain where an internal snapshot parent has an external
snapshot child, we lacked a safety check when trying to use the
--children-only option to snapshot-delete:

$ virsh start dom
$ virsh snapshot-create-as dom internal
$ virsh snapshot-create-as dom external --disk-only
$ virsh snapshot-delete dom external
error: Failed to delete snapshot external
error: unsupported configuration: deletion of 1 external disk snapshots not supported yet
$ virsh snapshot-delete dom internal --children
error: Failed to delete snapshot internal
error: unsupported configuration: deletion of 1 external disk snapshots not supported yet
$ virsh snapshot-delete dom internal --children-only
Domain snapshot internal children deleted

While I'd still like to see patches that actually do proper external
snapshot deletion, we should at least fix the inconsistency in the
meantime.  With this patch:

$ virsh snapshot-delete dom internal --children-only
error: Failed to delete snapshot internal
error: unsupported configuration: deletion of 1 external disk snapshots not supported yet

* src/qemu/qemu_driver.c (qemuDomainSnapshotDelete): Fix condition.
Signed-off-by: NEric Blake <eblake@redhat.com>

src/qemu/qemu_driver.c

@@ -14812,7 +14812,8 @@ qemuDomainSnapshotDelete(virDomainSnapshotPtr snapshot,
         if (!(flags & VIR_DOMAIN_SNAPSHOT_DELETE_CHILDREN_ONLY) &&
             virDomainSnapshotIsExternal(snap))
             external++;
-        if (flags & VIR_DOMAIN_SNAPSHOT_DELETE_CHILDREN)
+        if (flags & (VIR_DOMAIN_SNAPSHOT_DELETE_CHILDREN |
+                     VIR_DOMAIN_SNAPSHOT_DELETE_CHILDREN_ONLY))
             virDomainSnapshotForEachDescendant(snap,
                                                qemuDomainSnapshotCountExternal,
                                                &external);