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)".

docs/formatdomain.html.in

@@ -1703,7 +1703,7 @@
           <td>hidden</td>
           <td>Hide the KVM hypervisor from standard MSR based discovery</td>
           <td>on, off</td>
-          <td><span class="since">2.1.0 (QEMU only)</span></td>
+          <td><span class="since">1.2.8 (QEMU 2.1.0)</span></td>
         </tr>
       </table>
       </dd>