ch-remote: Fix crash when run with no subcommand

ch-remote crashes when run with --api-socket but no subcommand:

  $ target/release/ch-remote --api-socket /tmp/api
  thread 'main' panicked at src/bin/ch-remote.rs:509:14:
  internal error: entered unreachable code

Use clap::Command::subcommand_required(true) to yield a more friendly
error in this case.

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.")
+        .subcommand_required(true)
         .args([
             Arg::new("api-socket")
                 .long("api-socket")