perf kvm: introduce --list-cmds for use by scripts

Introduce

  $ perf kvm --list-cmds

to dump a raw list of commands for use by the completion script. To do
this, introduce build_usage_string() and list_cmds_raw(), and expose
them via util.h.

Cc: David Ahern <dsahern@gmail.com>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>