Split VNC client steps in two for aarch64 timing issue
    
    So, there's a problem with how we figure out the NetworkManager
    connection to use in setup_tap_static: it expects the first
    connection in the list to be the right one, but this is only
    actually true so long as it's *active*. When we're in the tap
    case, it's usually not going to actually *work* out of the box
    on boot (or else we wouldn't need setup_tap_static at all...),
    so some time after boot, NetworkManager gives up on it and marks
    it as inactive. And after that, setup_tap_static won't work any
    more.
    
    I never noticed this as a problem before because usually we do
    setup_tap_static before that point. But it seems in the vnc
    client tests, on aarch64, desktop boot and login is slow enough
    that by the time we switch to a VT and try to setup the network,
    we're very close to that cutoff, and sometimes miss it.
    
    This, I hope, avoids the problem by doing the network setup in
    that test before we deal with the desktop login, then doing the
    desktop login, then doing the actual VNC bits.
    
    The alternative here would be to figure out a better way to do
    setup_tap_static, but I can't.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>