Add new 'kvm' domain feature and ability to hide KVM signature

Add new 'kvm' domain feature and ability to hide KVM signature

QEMU 2.1 added support for the kvm=off option to the -cpu command,
allowing the KVM hypervisor signature to be hidden from the guest.
This enables disabling of some paravirualization features in the
guest as well as allowing certain drivers which test for the
hypervisor to load.  Domain XML syntax is as follows:

<domain type='kvm>
  ...
  <features>
    ...
    <kvm>
      <hidden state='on'/>
    </kvm>
  </features>
  ...

Signed-off-by: Alex Williamson <alex.williamson@redhat.com>