s390: make kvm_stat work on s390

Add s390_exit_reasons so kvm_stat doesn't crash when called on s390.
Look for 'vendor_id' in /proc/cpuinfo as well, instead of just for
'flags', so we can determine if we run on S390.

Signed-off-by: Jens Freimann <jfrei@linux.vnet.ibm.com>
Signed-off-by: Alexander Graf <agraf@suse.de>
