cli: polish CLI usage patterns

[guybedford]

This polishes the CLI arguments handling and basic usage.

CLI help with this change:

WASI Virt

Usage: wasi-virt [OPTIONS] --out <virt.wasm> [component.wasm]

Arguments:
  [component.wasm]  Wasm binary to compose the virtualization into.
                    If not provided, only the virtualization component itself will be generated,
                    which can then be composed via `wasm-tools compose -d virt.wasm component.wasm`

Options:
  -o, --out <virt.wasm>  Output virtualization component Wasm file
      --allow-all        Enable all subsystem passthrough (encapsulation is the default)
      --allow-clocks     Enable clocks
      --allow-exit       Allow the component to exit
      --allow-http       Enable HTTP
      --allow-random     Enable Random
      --allow-sockets    Enable Sockets
  -h, --help             Print help
  -V, --version          Print version

Env:
      --allow-env[=<ENV_VAR>...]  Allow unrestricted access to host environment variables, or to a comma-separated list of variable names
  -e, --env <ENV=VAR>             Set environment variable overrides

Fs:
      --allow-fs                       Allow unrestricted access to host preopens
      --mount <preopen=virtualdir>     Mount a virtual directory globbed from the local filesystem
      --preopen <preopen=hostpreopen>  Configure runtime preopen mappings

Stdio:
      --allow-stdio     Enable all stdio
      --stdio[=<cfg>]   Configure all stdio [possible values: allow, ignore, deny]
      --stderr[=<cfg>]  Configure stderr [possible values: allow, ignore, deny]
      --stdin[=<cfg>]   Configure stdin [possible values: allow, ignore, deny]
      --stdout[=<cfg>]  Configure stdout [possible values: allow, ignore, deny]