perf list: Print matching PMU events for --unit

When --unit option is used, pmu_glob is set to the argument. It should
match with event PMU and display the matching ones only. But it also
shows raw events and metrics after that.

$ perf list --unit tool
List of pre-defined events (to be used in -e or -M):

tool:
core_wide
[1 if not SMT,if SMT are events being gathered on all SMT threads 1 otherwise 0. Unit: tool]
duration_time
[Wall clock interval time in nanoseconds. Unit: tool]
has_pmem
[1 if persistent memory installed otherwise 0. Unit: tool]
num_cores
[Number of cores. A core consists of 1 or more thread,with each thread being associated with a logical Linux CPU. Unit: tool]
num_cpus
[Number of logical Linux CPUs. There may be multiple such CPUs on a core. Unit: tool]
...
rNNN [Raw event descriptor]
cpu/event=0..255,pc,edge,.../modifier [Raw event descriptor]
[(see 'man perf-list' or 'man perf-record' on how to encode it)]
breakpoint//modifier [Raw event descriptor]
cstate_core/event=0..0xffffffffffffffff/modifier [Raw event descriptor]
cstate_pkg/event=0..0xffffffffffffffff/modifier [Raw event descriptor]
drm_i915//modifier [Raw event descriptor]
hwmon_acpitz//modifier [Raw event descriptor]
hwmon_ac//modifier [Raw event descriptor]
hwmon_bat0//modifier [Raw event descriptor]
hwmon_coretemp//modifier [Raw event descriptor]
...

Metric Groups:

Backend: [Grouping from Top-down Microarchitecture Analysis Metrics spreadsheet]
tma_core_bound
[This metric represents fraction of slots where Core non-memory issues were of a bottleneck]
tma_info_core_ilp
[Instruction-Level-Parallelism (average number of uops executed when there is execution) per thread (logical-processor)]
tma_info_memory_l2mpki
[L2 cache true misses per kilo instruction for retired demand loads]
...

This change makes it print the tool PMU events only.

Reviewed-by: Ian Rogers <irogers@google.com>
Signed-off-by: Namhyung Kim <namhyung@kernel.org>