x86/hvm/domain: remove the 'hap_enabled' flag

x86/hvm/domain: remove the 'hap_enabled' flag

The hap_enabled() macro can determine whether the feature is available
using the domain 'options'; there is no need for a separate flag.

NOTE: Furthermore, by extending sanitizing of the domain 'options', the
      macro can be transformed into an inline function and re-located to
      xen/sched.h. This also makes hap_enabled() common, thus allowing
      removal of an ugly ifdef CONFIG_X86 from the common iommu code.

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>