Merge r216365 from head to stable/8:

Merge r216365 from head to stable/8:

  Derive the XENHVM kernel from GENERIC, adding only the options required
  to support PV drivers (such as xenpci), and non-adptive locking (along
  with a comment about why).

  This change eliminates the synchronisation problem between GENERIC and
  XENHVM, which had become severely rotted in HEAD, and in 8-STABLE
  included non-production kernel debugging features such as WITNESS.

  However, it comes at the cost of enabling devices and options that may
  not be present under Xen (such as random ethernet cards).  For now, opt
  for a simpler kernel configuration file rather than using nooptions/
  nodevice to enumerate and eliminate them.  This leads to a somewhat
  larger XENHVM kernel.

  This is an MFC candidate for 8-STABLE before 8.2, in order to provide
  a production-worthy XENHVM kernel configuration for amd64.

  Discussed with:       gibbs, cperciva
  Reported by:          Piete Brooks <Piete.Brooks at cl.cam.ac.uk>
  Sponsored by:         DARPA, AFRL

Approved by:	re (kib)

git-svn-id: svn://svn.freebsd.org/base/stable/8@216406 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f