virt: Override default nettype based on uid

When test runner is executed as root, default to using bridge-mode
networking.  Otherwise, force it to user-mode, unless different
on the command line.

Signed-off-by: Chris Evich <cevich@redhat.com>
virt: Warn if uid==0 and nettype==user

Sample output:

15:37:08 INFO | Starting test ...
15:37:08 DEBUG| Test parameters:
                     ...
15:39:52 WARNI|
15:39:52 WARNI| Testing with nettype='user' while running as root may
                produce unexpected results!!!
15:39:52 WARNI|

Signed-off-by: Chris Evich <cevich@redhat.com>