main: return error on invalid length of arguments
main: return error on invalid length of arguments

For the commands that verify the length of arguments, returning
Ok on an invalid length will confuse any external programs that
launch crosvm because they still get exit code of success from
crosvm even when the command failed.

Also add a missed return of 'create_qcow2' sub-command.

BUG=None
TEST=some basic tests:
() cargo check
() verify the return value from changed sub-commands is not 0 with
an invalid argument length.
() launch a VM with concierge_client from a root shell

Change-Id: I8278107a4d2fcf3cb6fafb65f30f431f97f7deb1
Signed-off-by: Jianxun Zhang <cf37b857570b0701d39fd0feb77f6a52483b5c8d@intel.com>
Reviewed-on: https://chromium-review.googlesource.com/1501552
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Zach Reizner <3374927928a53d04457674c546bd4de57c2044ec@chromium.org>

google/crosvm,google/crosvm,google/crosvm,google/crosvm