fix(console): a serial console reports its VM's stop

When a VM stopped, its console's stream could end before the worker's
next check saw the VM shut off. The check then found no stream to drop and
said nothing, or, seeing the VM still running, opened the console again,
which libvirt refused, and the session ended. The worker now remembers
whether the VM ran at its last check, and a console that cannot be opened
again because the VM stopped meanwhile is a stop: the terminal says the VM
is not running and the console opens again when it starts. A new stream is
the console's before its callback is registered, which may end it at once.

The live serial test's VM now repeats its BIOS banner (it reboots right
after finding no boot disk), so a console attached late still gets one,
and the test waits for the superuser's console to show it before the VM
stops. A run of the whole live suite over the socket missed the banner of
the paused-then-resumed VM before.

conf/daemon/consolecallback

@@ -47,19 +47,28 @@ class Console:
         self.domain = connection.lookupByUUIDString(uuid)
         self.connection = connection
         self.stream = None
+        self.active = False  # the VM ran at the last check
         self.running = True
 
     def check(self):
-        """Open the stream while the VM runs, drop it while it does not."""
+        """Open the stream while the VM runs, drop it while it does not. The
+        stream may have ended first: the VM's stop is still reported."""
         active = self.domain.state()[0] in (libvirt.VIR_DOMAIN_RUNNING, libvirt.VIR_DOMAIN_PAUSED)
         if active and self.stream is None:
             stream = self.connection.newStream(libvirt.VIR_STREAM_NONBLOCK)
-            self.domain.openConsole(None, stream, 0)  # a console in use is refused
-            stream.eventAddCallback(libvirt.VIR_STREAM_EVENT_READABLE, from_vm, self)
-            self.stream = stream
-        elif not active and self.stream is not None:
+            try:
+                self.domain.openConsole(None, stream, 0)  # a console in use is refused
+            except libvirt.libvirtError:
+                if self.domain.isActive():
+                    raise
+                active = False  # it stopped meanwhile
+            else:
+                self.stream = stream  # first: the callback may end it at once
+                stream.eventAddCallback(libvirt.VIR_STREAM_EVENT_READABLE, from_vm, self)
+        if not active and self.active:
             self.close_stream()
             say("The VM is not running: its console opens again when it starts")
+        self.active = active
 
     def close_stream(self):
         stream, self.stream = self.stream, None