cli: only show subcommand specific usage if command incomplete

cli: only show subcommand specific usage if command incomplete

before:
$ pveum acl
ERROR: incomplete command 'pveum acl'
USAGE: pveum <COMMAND> [ARGS] [OPTIONS]
       pveum acl delete <path> --roles <string> [OPTIONS]
       pveum acl modify <path> --roles <string> [OPTIONS]

       pveum group add <groupid> [OPTIONS]
       pveum group delete <groupid>
       pveum group modify <groupid> [OPTIONS]

       pveum role add <roleid> [OPTIONS]
       pveum role delete <roleid>
       pveum role modify <roleid> --privs <string> [OPTIONS]

       pveum user add <userid> [OPTIONS]
       pveum user delete <userid>
       pveum user modify <userid> [OPTIONS]

       pveum passwd <userid>
       pveum ticket <username> [OPTIONS]

       pveum help [<extra-args>] [OPTIONS]

after:
$ pveum acl
ERROR: incomplete command 'pveum acl'
USAGE: pveum <COMMAND> [ARGS] [OPTIONS]
       pveum acl delete <path> --roles <string> [OPTIONS]
       pveum acl modify <path> --roles <string> [OPTIONS]

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>