perf kvm: Fix subcommands on s390

perf kvm: Fix subcommands on s390

With commit eca0fa28cd0d ("perf record: Provide detailed information on
s390 CPU") s390 platform provides detailed type/model/capacity
information in the CPU identifier string instead of just "IBM/S390".

This breaks 'perf kvm' support which uses hard coded string IBM/S390 to
compare with the CPU identifier string. Fix this by changing the
comparison.

Reported-by: Stefan Raspl <raspl@linux.ibm.com>
Signed-off-by: Thomas Richter <tmricht@linux.ibm.com>
Reviewed-by: Hendrik Brueckner <brueckner@linux.ibm.com>
Tested-by: Stefan Raspl <raspl@linux.ibm.com>
Acked-by: Christian Borntraeger <borntraeger@de.ibm.com>
Cc: Heiko Carstens <heiko.carstens@de.ibm.com>
Cc: Martin Schwidefsky <schwidefsky@de.ibm.com>
Cc: stable@vger.kernel.org
Fixes: eca0fa28cd0d ("perf record: Provide detailed information on s390 CPU")
Link: http://lkml.kernel.org/r/20180712070936.67547-1-tmricht@linux.ibm.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>