vmm: Support setting seccomp to errno

This will generate -EPERM on seccomp violations as opposed to causing
the VMM to exit with SIGSYS.

Signed-off-by: Rob Bradford <rbradford@meta.com>

cloud-hypervisor/src/main.rs

@@ -454,7 +454,7 @@ fn get_cli_options_sorted(
         Arg::new("seccomp")
             .long("seccomp")
             .num_args(1)
-            .value_parser(["true", "false", "log"])
+            .value_parser(["true", "false", "log", "errno"])
             .default_value("true"),
         Arg::new("serial")
             .long("serial")
@@ -588,6 +588,7 @@ fn start_vmm(
             "true" => SeccompAction::Trap,
             "false" => SeccompAction::Allow,
             "log" => SeccompAction::Log,
+            "errno" => SeccompAction::Errno(libc::EPERM as u32),
             val => {
                 // The user providing an invalid value will be rejected
                 panic!("Invalid parameter {val} for \"--seccomp\" flag");