virsh_net_autostart: Rework test to not use 'default'

Reworked the code to not use the 'default' network bridge since
doing so may result in issues with the target system expectations
that virbr0 (the 'default' network bridge) exists and is running
when the libvirtd is restarted as part of the test.

Instead, create our own network bridge using a new configuration
option 'net_autostart_net_name', then apply the same logic regarding
being able to set or disable the autostart flag.