[product: zaphod][project: zaphod-v0.3]: Implemented _networking_mode

SYNOPSIS
[modified]m42/test/test_zaphod.py: Implemented test__networking_mode
[modified]m42/test/vmguard.py: Added support for bridged mode.
[modified]m42/test/zaphodtestutil.py: createvm supports bridged mode.
[modified]m42/zaphod.py: Implemented _networking_mode

DETAILS
Implemented method _networking_mode in zaphod.py. To test this
requires that zpahodtestutil.createvm support the creation of with
bridged mode networking. 'Backported' the implementation of bridged
mode networking from zaphod.createvm to zaphodtestutil.py. Other
changes in the code can be attributed to changes necessitated by the
change to the function signature of zaphodtestutil.createvm.