Most hypervisors use Hardware Assisted Paging by default and don't
require specifying the feature in domain conf. But some hypervisors
support disabling HAP on a per-domain basis. To enable HAP by default
yet provide a knob to disable it, extend the <hap> feature with a
'state=on|off' attribute, similar to <pvspinlock> and <vmport> features.

In the absence of <hap>, the hypervisor default (on) is used. <hap>
without the state attribute would be the same as <hap state='on'/> for
backwards compatibility. And of course <hap state='off'/> disables hap.
Signed-off-by: NJim Fehlig <jfehlig@suse.com>