collie: print subcommand usage when function return EXIT_USAGE

Output before:
$collie vdi create test
Please specify the VDI size

Output after:
$collie vdi create test
Please specify the VDI size
Usage: collie vdi create [-P] [-c copies] [-a address] [-p port] [-h] <vdiname> <size>
Options:
-P, --prealloc          preallocate all the data objects
-c, --copies            specify the data redundancy (number of copies)
-a, --address           specify the daemon address (default: localhost)
-p, --port              specify the daemon port
-h, --help              display this help and exit

Signed-off-by: Kai Zhang <kyle@zelin.io>
Signed-off-by: MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>