fix <hypervisor/> to preclude oVirt

Putting <hypervisor value="" /> in a recipe's hostRequires should
prevent it from being run on an oVirt guest, but that wasn't working due
to this surprising little gotcha:

    >>> 'KVM'.__eq__(None)
    NotImplemented
    >>> bool('KVM'.__eq__(None))
    True
    >>> operator.__eq__('KVM', None)
    False

Change-Id: Id45d1dd45141d5e7ff8d1ddb7be9da09413fe3b6
Merge "fix <hypervisor/> to preclude oVirt" into release-0.12