Fix fallouts from Linux header inclusion

Fix fallouts from Linux header inclusion

This is an all-in-one fix for the smaller and bigger mistakes of the
build system changes for accompanied Linux headers:
 - only enable KVM and vhost on Linux hosts
 - fix powerpc asm header symlink
 - do not use Linux headers on non-Linux hosts
 - fix kvmclock for !CONFIG_KVM
 - fix s390 build on non-Linux hosts

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Tested-by: Andreas Färber <andreas.faerber@web.de>
Tested-by: Stefan Weil <weil@mail.berlios.de>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>