X86: Add a check to catch Xen emulation of Hyper-V

X86: Add a check to catch Xen emulation of Hyper-V

Xen emulates Hyper-V to host enlightened Windows. Looks like this
emulation may be turned on by default even for Linux guests. Check and
fail Hyper-V detection if we are on Xen.

[ hpa: the problem here is that Xen doesn't emulate Hyper-V well
  enough, and if the Xen support isn't compiled in, we end up stubling
  over the Hyper-V emulation and try to activate it -- and it fails. ]

Signed-off-by: K. Y. Srinivasan <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: H. Peter Anvin <[email protected]>