qemu: Handle host devices not being available better
    
    We can't retrieve the isolation group of a device that's not present
    in the system. However, it's very common for VFs to be created late
    in the boot, so they might not be present yet when libvirtd starts,
    which would cause the guests using them to disappear.
    
    Moreover, for other architectures and even ppc64 before isolation
    groups were introduced, it's considered perfectly fine to configure a
    guest to use a device that's not yet (or no longer) available to the
    host, with the obvious caveat that such a guest won't be able to
    start before the device is available.
    
    In order to be consistent, when a device's isolation group can't be
    determined fall back to not isolating it rather than erroring out or,
    worse, making the guest disappear.
    
    Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1484254
    
    Signed-off-by: Andrea Bolognani <abologna@redhat.com>