monitor: support sub command in auto completion

monitor: support sub command in auto completion

This patch allows auto completion work normal for sub command case,
"info block [DEVICE]" can auto complete now, by re-enter the completion
function. In original code "info" is treated as a special case, now it
is treated as a sub command group, global variable info_cmds is not used
any more.

"help" command is still treated as a special case, since it is not a sub
command group but want to auto complete command in root command table.

Signed-off-by: Wenchao Xia <[email protected]>
Reviewed-by: Eric Blake <[email protected]>
Signed-off-by: Luiz Capitulino <[email protected]>