perf kvm: Use macro to replace variable 'decode_str_len'

perf kvm: Use macro to replace variable 'decode_str_len'

The variable 'decode_str_len' defines the string length for KVM event
name and every arch defines its own values.

This introduces complexity that the variable definition are spreading in
multiple source files under arch folder.  This patch refactors code to
use a macro KVM_EVENT_NAME_LEN to define event name length and thus
remove the definitions in arch files.

Signed-off-by: Leo Yan <leo.yan@linaro.org>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: James Clark <james.clark@arm.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: John Garry <john.g.garry@oracle.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: linux-arm-kernel@lists.infradead.org
Link: https://lore.kernel.org/r/20230315145112.186603-2-leo.yan@linaro.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>