qemuSnapshotPrepare: Prohibit 'manual' disk snapshot mode with VIR_DOMAIN_SNAPSHOT_CREATE_QUIESCE flag

If the snapshot has a disk using 'manual' snapshot mode we keep the VM
paused until the user resumes it (presumably after they've done steps to
take the disk snapshot).

Since quiescing is done via the guest agent this means it will not be
possible while the VM is paused.

Rather than trying to implement complex recovery from this state prevent
the use of VIR_DOMAIN_SNAPSHOT_CREATE_QUIESCE for the snapshot. The user
still can call virDomainFSFreeze/virDomainFSThaw manually.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>

src/qemu/qemu_snapshot.c

@@ -1075,6 +1075,12 @@ qemuSnapshotPrepare(virDomainObj *vm,
         }
     }
 
+    if (*has_manual && (*flags & VIR_DOMAIN_SNAPSHOT_CREATE_QUIESCE)) {
+            virReportError(VIR_ERR_OPERATION_UNSUPPORTED, "%s",
+                           _("'manual' disk snapshot mode requires explicit quiescing (VIR_DOMAIN_SNAPSHOT_CREATE_QUIESCE is not supported)"));
+            return -1;
+    }
+
     /* Alter flags to let later users know what we learned.  */
     if (external && !active)
         *flags |= VIR_DOMAIN_SNAPSHOT_CREATE_DISK_ONLY;