OvmfPkg: QemuVideoDxe: disentangle UEFI driver model use in Stop()
    
    A bus driver needs to pay attention whether its Stop() function is being
    called on the "main" controller handle (NumberOfChildren == 0) or on the
    child handles (NumberOfChildren > 0).
    
    In QemuVideoDxe, all our resources are associated with the one child
    handle (and the Private data structure) *except* the top-level PciIo
    protocol reference. Be conscious of which mode Stop() is being called for.
    
    Contributed-under: TianoCore Contribution Agreement 1.0
    Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
    Signed-off-by: Laszlo Ersek <lersek@redhat.com>