docs: correct version requirements for <kvm><hidden='on'/></kvm>
    
    When support was added for the kvm hidden='on' attribute in commit
    d07116, the version requirement was listed as "2.1.0 (QEMU
    only)". However, this was added when libvirt was at version 1.2.8 - it
    is *QEMU* that must be at version 2.1.0 or later.
    
    This went unnoticed for a very long time (over 2 years). Then a week
    or two ago a new Windows convert in the #virt channel on OFTC was told
    he needed to use this feature (to prevent nvidia drivers in a guest
    from refusing to work due to being run in a virtual machine). There
    was some problem with it being recognized and "someone" (it may have
    been me, or may have been someone else, I don't remember) pointed out
    that the documentation at
    
      http://www.libvirt.org/formatdomain.html
    
    says that it requires libvirt 2.1.0. The next several days were filled
    with agony as a new convert to Linux first tried to upgrade a Linux
    Mint host running their "LTS" version to something newer, then tried
    to install a libvirt build built for Ubuntu onto this, and later back
    to the old LTS Linux Mint. After this he tried building his own
    libvirt from source (with all the expected problems), and finally
    switched to Fedora. In the end it was hours and hours of everybody's
    lives that they will never get back. To now learn that he didn't need
    to do this (his original libvirt version was 1.3.3, so whatever his
    problem was, it was elsewhere) makes the pain all that much worse.
    
    To prevent this from happening again, this simple patch changes the
    version requirement for the kvm hidden attribute from "2.1.0 (QEMU
    only)" to "1.2.8 (QEMU 2.1.0)".