Add quotes around the register name and suggest using 'perf record -I?'
to get the list of available registers.

Before:

  # perf record -Idi,xmm20,xmm1
  Warning:
  unknown register xmm20, check man page

   Usage: perf record [<options>] [<command>]
      or: perf record [<options>] -- <command> [<options>]

      -I, --intr-regs[=<any register>]
                            sample selected machine registers on interrupt, use -I ? to list register names
  #
  # perf record -Idi,xmm20,xmm1
  Warning:
  unknown register "xmm20", check man page or run "perf record -I?"

   Usage: perf record [<options>] [<command>]
      or: perf record [<options>] -- <command> [<options>]

      -I, --intr-regs[=<any register>]
                            sample selected machine registers on interrupt, use -I ? to list register names
  #

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Kan Liang <kan.liang@linux.intel.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Link: https://lkml.kernel.org/n/tip-9a9hyuum8c0oggg86xd3sxc5@git.kernel.orgSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>