include/exec/poison: Mark CONFIG_KVM as poisoned, too

CONFIG_KVM is only defined for target-specific code, so nobody should
use it by accident in common code. To avoid such subtle bugs,
CONFIG_KVM is now marked as poisoned in common code. The header
include/sysemu/kvm.h is somewhat special since it is included
all over the place from common code, too, so we need some extra
logic via "#ifdef NEED_CPU_H" here to make sure that we can
compile all files without problems.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-Id: <1498454578-18709-4-git-send-email-thuth@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
