intel idle: Make idle driver more robust

kvm -cpu host passes the original cpuid info to the guest.

Latest kvm version seem to return true for mwait_leaf cpuid
function on recent Intel CPUs. But it does not return mwait
C-states (mwait_substates), instead zero is returned.

While real CPUs seem to always return non-zero values, the intel
idle driver should not get active in kvm (mwait_substates == 0)
case and bail out.
Otherwise a Null pointer exception will happen later when the
cpuidle subsystem tries to get active:
[0.984807] BUG: unable to handle kernel NULL pointer dereference at (null)
[0.984807] IP: [<(null)>] (null)
...
[0.984807][<ffffffff8143cf34>] ? cpuidle_idle_call+0xb4/0x340
[0.984807][<ffffffff8159e7bc>] ? __atomic_notifier_call_chain+0x4c/0x70
[0.984807][<ffffffff81001198>] ? cpu_idle+0x78/0xd0

Reference:
https://bugzilla.novell.com/show_bug.cgi?id=726296



Cc: stable@vger.kernel.org
Signed-off-by: Thomas Renninger <trenn@suse.de>
CC: Bruno Friedmann <bruno@ioda-net.ch>
Signed-off-by: Len Brown <len.brown@intel.com>