perf test stat tests: Fix for virtualized machines

On s390 'perf test's 'perf stat tests', subtest test_hybrid fails for
z/VM systems. The root cause is this statement:

$(perf stat -a -- sleep 0.1 2>&1 |\
grep -E "/cpu-cycles/[uH]*| cpu-cycles[:uH]* -c)

The 'perf stat' output on a s390 z/VM system is

# perf stat -a -- sleep 0.1 2>&1
Performance counter stats for 'system wide':

56 context-switches # 46.3 cs/sec cs_per_second
1,210.41 msec cpu-clock # 11.9 CPUs CPUs_utilized
12 cpu-migrations # 9.9 migrations/sec ...
81 page-faults # 66.9 faults/sec ...

0.100891009 seconds time elapsed

The grep command does not match any single line and exits with error
code 1.

As the bash script is executed with 'set -e', it aborts with the first
error code being non-zero.

Fix this and use 'wc -l' to count matching lines instead of 'grep ... -c'.

Output before:

# perf test 102
102: perf stat tests : FAILED!
#

Output after:

# perf test 102
102: perf stat tests : Ok
#

Fixes: bb6e7cb11d97ce19 ("perf tools: Add fallback for exclude_guest")
Reviewed-by: Ian Rogers <irogers@google.com>
Reviewed-by: James Clark <james.clark@linaro.org>
Signed-off-by: Thomas Richter <tmricht@linux.ibm.com>
Cc: Alexander Gordeev <agordeev@linux.ibm.com>
Cc: Heiko Carstens <hca@linux.ibm.com>
Cc: Jan Polensky <japo@linux.ibm.com>
Cc: linux-s390@vger.kernel.org
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Sumanth Korikkar <sumanthk@linux.ibm.com>
Cc: Vasily Gorbik <gor@linux.ibm.com>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>