perf test 6: Fix missing kvm module load for s390

Command

   # perf test -Fv 6

fails with error

   running test 100 'kvm-s390:kvm_s390_create_vm' failed to parse
    event 'kvm-s390:kvm_s390_create_vm', err -1, str 'unknown tracepoint'
    event syntax error: 'kvm-s390:kvm_s390_create_vm'
                         \___ unknown tracepoint

when the kvm module is not loaded or not built in.

Fix this by adding a valid function which tests if the module
is loaded. Loaded modules (or builtin KVM support) have a
directory named
  /sys/kernel/debug/tracing/events/kvm-s390
for this tracepoint.

Check for existence of this directory.

Signed-off-by: Thomas Richter <tmricht@linux.ibm.com>
Reviewed-by: Christian Borntraeger <borntraeger@de.ibm.com>
Cc: Heiko Carstens <heiko.carstens@de.ibm.com>
Cc: Hendrik Brueckner <brueckner@linux.vnet.ibm.com>
Link: http://lkml.kernel.org/r/20190604053504.43073-1-tmricht@linux.ibm.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>