MFC r228746

MFC r228746
  The Xen pmap doesn't support superpages.  So, there is no point in it
  initializing structures, like the pv table, that are only used to
  implement superpages.  In fact, some of the unnecessary code in
  pmap_init() was actually doing harm.  It was preventing the kernel from
  booting on virtual machines with more than 768 MB of memory.

Note: The change to pmap_page_is_mapped() differs slightly from r228746
  because of differences in how the page queues lock is used in
  FreeBSD 8.x.

git-svn-id: svn://svn.freebsd.org/base/stable/8@230433 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f