fix(instances): a new VM and each clone get a VNC password of their own

Since the VM create became a job (6f0c7300) every new VM was defined with
no VNC password, and a clone copied its source's password (or its lack
of one) exactly: every clone of a template shared it. Computes listen
for VNC on all their addresses (dev/libvirt-bootstrap.sh), so anyone who
could reach a compute's VNC ports opened any of these consoles directly,
without the panel; the README's firewall rule was the only guard.

The create job asks for create.py's random 8-character password again,
and a clone's VNC graphics get a new random one. The console page sends
the password to a user with passwordless_console, which the user create
form now ticks by default next to change_password; a user without it is
asked for the password, as the permission intends. Existing VMs keep
their settings.

Live create, clone and VM page tests pass on a TCP compute.

vrtManager/instance.py

@@ -2398,6 +2398,10 @@ class wvmInstance(wvmConnect):
                 raise ValueError(f"No MAC address for network interface {num}")
             net.find("mac").set("address", self.fix_mac(mac))
 
+        # the clone's console gets a password of its own: one copied from the source would open both
+        for graphics in tree.findall("devices/graphics[@type='vnc']"):
+            graphics.set("passwd", util.randomPasswd(8))  # the length VNC allows
+
         nvram = tree.find("os/nvram")
         if nvram is not None and nvram.get("type", "file") != "file":
             raise util.OperationError(gettext_noop("Cannot clone a VM whose NVRAM is not a file"))