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).
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
MFC after:	3 days

jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase