remove xhat
The xhat infrastructure was added to support hardware such as the zulu
graphics card - hardware which had on-board MMUs.  The VM used the xhat code
to keep the CPU's and Zulu's page tables in-sync.  Since the only xhat user
was zulu (which is gone), we can safely remove it simplifying the whole VM
subsystem.
Assorted notes:
- AS_BUSY flag was used solely by xhat

*** 180,194 ****
                  return (0);
  
          /*
           * as->a_wpage can only be changed while the process is totally stopped.
           * Don't grab p_lock here.  Holding p_lock while grabbing the address
!          * space lock leads to deadlocks with the clock thread.  Note that if an
!          * as_fault() is servicing a fault to a watched page on behalf of an
!          * XHAT provider, watchpoint will be temporarily cleared (and wp_prot
!          * will be set to wp_oprot).  Since this is done while holding as writer
!          * lock, we need to grab as lock (reader lock is good enough).
           *
           * p_maplock prevents simultaneous execution of this function.  Under
           * normal circumstances, holdwatch() will stop all other threads, so the
           * lock isn't really needed.  But there may be multiple threads within
           * stop() when SWATCHOK is set, so we need to handle multiple threads
--- 180,190 ----
                  return (0);
  
          /*
           * as->a_wpage can only be changed while the process is totally stopped.
           * Don't grab p_lock here.  Holding p_lock while grabbing the address
!          * space lock leads to deadlocks with the clock thread.
           *
           * p_maplock prevents simultaneous execution of this function.  Under
           * normal circumstances, holdwatch() will stop all other threads, so the
           * lock isn't really needed.  But there may be multiple threads within
           * stop() when SWATCHOK is set, so we need to handle multiple threads
*** 502,512 ****
          int rv;
  
          if ((as == &kas) || avl_numnodes(&as->a_wpage) == 0)
                  return (0);
  
-         /* Grab the lock because of XHAT (see comment in pr_mappage()) */
          AS_LOCK_ENTER(as, &as->a_lock, RW_READER);
          rv = pr_is_watchpage_as(addr, rw, as);
          AS_LOCK_EXIT(as, &as->a_lock);
  
          return (rv);
--- 498,507 ----