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

*** 1682,1693 ****
          for (sfhmep = pp->p_mapping; sfhmep; sfhmep = tmphme) {
                  tmphme = sfhmep->hme_next;
                  if (IS_PAHME(sfhmep))
                          continue;
                  hmeblkp = sfmmu_hmetohblk(sfhmep);
-                 if (hmeblkp->hblk_xhat_bit)
-                         continue;
                  tmphat = hblktosfmmu(hmeblkp);
                  sfmmu_copytte(&sfhmep->hme_tte, &tte);
                  ASSERT(TTE_IS_VALID(&tte));
                  if ((tmphat == ksfmmup) || hmeblkp->hblk_lckcnt) {
                          /*
--- 1682,1691 ----
*** 1707,1718 ****
          for (sfhmep = pp->p_mapping; sfhmep; sfhmep = tmphme) {
                  tmphme = sfhmep->hme_next;
                  if (IS_PAHME(sfhmep))
                          continue;
                  hmeblkp = sfmmu_hmetohblk(sfhmep);
-                 if (hmeblkp->hblk_xhat_bit)
-                         continue;
                  (void) sfmmu_pageunload(pp, sfhmep, TTE8K);
          }
  
          /*
           * Unloads only does tlb flushes so we need to flush the
--- 1705,1714 ----