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Build Identifier: 

When performing a clean install from the DVD image on a USB, in a 'server'
configuration with the 'xen host' package bundle, and a non-trivial set of
network configurations, and the installer option 'propose a bridge for virtual
machines' enabled, the installer emits a ruby 'undefined method connected?' for
nil:NilClass'.

The full ruby stack-trace can be found in the attached y2 logs bundle in
YaST2/y2log around line 79249, the YaST2 caller was:
/mounts/mp_0001/usr/share/YaST2/lib/y2network/virtualization_config.rb:95:in
`connected_and_bridgeable?' making me suspect it's related to the algorithm
attempting to propose a bridge device. (Which suggests a potential workaround
as disabling the 'propose a bridge' installer option).

During the earlier installer phase, I set up the network configuration as a
VLAN interface with static IP, stacked on top of a bond in LACP mode, which was
stacked on top of 2 RTL816x physical interfaces - of which only one had a cable
attached to the switch at the point of running the installer.

Reproducible: Didn't try

Steps to Reproduce:
I haven't confirmed but suspect this will do:
1. Run 15.3 installer from USB
2. During network setup phase, create a bond on some physical devices, and a
VLAN on top of the bond device
3. During package selection phase, use the 'minimal server' + 'xen host'
bundles
4. Run the rest of the installer, and it will hopefully give the IE during the
'saving network configuration phase'.
Actual Results:  
Installation paused to open a YaST2 dialog reporting the internal error and a
request to file a bug report with the contents from save_y2logs, upon clicking
"no" to skip launching a debugger, the installation appeared to continue okay.

Expected Results:  
It should probably have requested user clarification on what device to use, if
the algorithm was unsure about a sensible VM bridge proposal.

The machine has 5 physical network interfaces (2x 1G RTL816x on the
motherboard, a third 1G RTL816x on a PCI-E card, and 2x 10G QLCxxx on a PCI-E
card), of which only one of the on-board 1G ports was connected to a switch
during the installation process, the rest were all N/C.