Do not exit if imc_log setting fails

This can happen on virtual guests, in this case vmware:
https://bugzilla.novell.com/show_bug.cgi?id=827616

[AK: Improve warning message]
Signed-off-by: Thomas Renninger <trenn@suse.de>
Signed-off-by: Andi Kleen <ak@linux.intel.com>
Return by default if no CPU matches in set_imc_log

Do not issue uninitialized msr values if set_imc_log is called on
unsupported CPUs.

This is code clean up (makes it more robust).

Signed-off-by: Thomas Renninger <trenn@suse.de>
Signed-off-by: Andi Kleen <ak@linux.intel.com>