serialcon: Fix opening with xen (bz 1215357)

Apparently xen doesn't set <serial> device alias names, and OpenConsole
just opens the first <console> device unconditionally.

Properly handling that would be to report it in the UI that we can
only open the first console, but for now just drop the alias check.
Apparently SUSE already does this as mentioned in the RH bug

https://bugzilla.redhat.com/show_bug.cgi?id=1215357

virtManager/serialcon.py

@@ -205,8 +205,9 @@ class LibvirtConsoleConnection(ConsoleConnection):
         name = dev and dev.alias.name or None
         logging.debug("Opening console stream for dev=%s alias=%s",
                       dev, name)
-        if not name:
-            raise RuntimeError(_("Cannot open a device with no alias name"))
+        # libxl doesn't set aliases, their open_console just defaults to
+        # opening the first console device, so don't force prescence of
+        # an alias
 
         stream = self.conn.get_backend().newStream(libvirt.VIR_STREAM_NONBLOCK)
         self.vm.open_console(name, stream)