Fixes Fedora 20 network issues when virtual

On Fedora 20 virtual machines biosdevname command
'Returns 4 if running in a virtual machine.'

This patch:
  - Uses the biosdevname command return value to detect if virtual
  - Uses /sys/net to get interface names - Might be better solution
  - Leaves unchanged the original 'bare metal' case - I wonder what for
    though?

Tested with Fedora Cloud image adapted for vagrant-libvirt

Fixes issue #4104
Merge pull request #4297 from gildub/bug/fedora20+network

guests/fedora: Fixes Fedora 20 network issues when virtual