tests: qemuxml2argv: drop redundant serial testing
    
    Several cases have incidental <serial> or <console> XML which aren't
    the features being tested for. Upcoming changes will cause some
    churn here, so instead drop these bits now.
    
    Reviewed-by: Andrea Bolognani <abologna@redhat.com>
    Signed-off-by: Cole Robinson <crobinso@redhat.com>