OK, I've tried various options to try to replicate your issue. Here's what I find.
My virbr1 device is the outsider VM network, with IP address 192.168.100.1
Normally, it's running, as confirmed by this output excerpt to the ifconfig virbr1
virbr1 Link encap:Ethernet HWaddr 52:54:00:35:84:FF
inet addr:192.168.100.1 Bcast:192.168.100.255 Mask:255.255.255.0
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
When it's down, I see this instead in the third line:
BROADCAST MULTICAST MTU:1500 Metric:1
In that case, in the VNC window that appears, I get the following error message:
failed to retrieve ftp://192.168.100.1/pub/ks/outsider1-ks.cfg:
Couldn't connect to server
If this is where you're seeing the connection problem, check for activity on your virtual network devices. If you do get the VNC window at all, click Send Key | Ctrl+Alt+F3 and Send Key | Ctrl+Alt+F4 for messages associated with the VM creation process.
Alternatively, if you get an error message at the command line, before any VNC window appears, it might look something like:
ERROR [Errno 111] Connection refused
ERROR [Errno 113] No route to host
(To indirectly answer your question about forcing network configuration -- make sure your libvirtd and libvirt-guests services are running. That also controls the virtual network devices.)