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>

src/bin/ch-remote.rs

@@ -898,6 +898,7 @@ fn main() {
         .author(env!("CARGO_PKG_AUTHORS"))
         .version(env!("BUILD_VERSION"))
         .about("Remotely control a cloud-hypervisor VMM.")
+        .arg_required_else_help(true)
         .subcommand_required(true)
         .args([
             Arg::new("api-socket")

src/main.rs

@@ -156,6 +156,7 @@ fn create_app(default_vcpus: String, default_memory: String, default_rng: String
         // compile time
         .author(env!("CARGO_PKG_AUTHORS"))
         .about("Launch a cloud-hypervisor VMM.")
+        .arg_required_else_help(true)
         .group(ArgGroup::new("vm-config").multiple(true))
         .group(ArgGroup::new("vmm-config").multiple(true))
         .group(ArgGroup::new("logging").multiple(true))

vhost_user_block/src/main.rs

@@ -20,6 +20,7 @@ fn main() {
         .version(env!("CARGO_PKG_VERSION"))
         .author(env!("CARGO_PKG_AUTHORS"))
         .about("Launch a vhost-user-blk backend.")
+        .arg_required_else_help(true)
         .arg(
             Arg::new("block-backend")
                 .long("block-backend")

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")