main: Show help text when run without arguments

cloud-hypervisor, ch-remote, vhost-user-block and vhost-user-net all
need at least one argument to do anything useful, so printing command
help is helpful when they are run without arguments or a subcommand.

Use clap::Command::arg_required_else_help(true) to do this.

Signed-off-by: Chris Webb <chris@arachsys.com>

vhost_user_net/src/main.rs

@@ -16,6 +16,7 @@ fn main() {
         .version(env!("CARGO_PKG_VERSION"))
         .author(env!("CARGO_PKG_AUTHORS"))
         .about("Launch a vhost-user-net backend.")
+        .arg_required_else_help(true)
         .arg(
             Arg::new("net-backend")
                 .long("net-backend")