x86: hap_enabled() is HVM-only

x86: hap_enabled() is HVM-only

There at least two cases where the field so far got accessed for PV
guests as well: One is in iommu_construct(), via iommu_use_hap_pt(),
and the other is
arch_domain_create()
-> paging_domain_init()
   -> p2m_init()
      -> p2m_init_hostp2m()
         -> p2m_init_one()
            -> p2m_initialise()
It just so happens that the field currently lives in struct hvm_domain
at an offset larger than sizeof(struct pv_domain).

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>