Add --gdb flag to vm run,run-app and run-microdroid commands

This flag will be passed through to crosvm, which in turn will start a
gdb server at the given port. This will allow us to attach gdb to guest
kernel.

There are following rules on how --gdb flag can be used:

1. Only caller with USE_CUSTOM_VIRTUAL_MACHINE permission (a.k.a. shell)
   can specify gdbPort in the payload config
2. The --gdb flag is only supported for non-protected VMs.
3. The --gdb flag is only supported for fully debuggable app payloads.

Bug: 242057159
Test: run-microdroid --gdb 3456
Change-Id: Iea93d8e6b37d23801d84cc1dc5fc1a250b54a047
diff --git a/rialto/tests/test.rs b/rialto/tests/test.rs
index 04a87f3..be5f118 100644
--- a/rialto/tests/test.rs
+++ b/rialto/tests/test.rs
@@ -68,6 +68,7 @@
         cpuTopology: CpuTopology::ONE_CPU,
         platformVersion: "~1.0".to_string(),
         taskProfiles: vec![],
+        gdbPort: 0, // No gdb
     });
     let vm = VmInstance::create(service.as_ref(), &config, Some(console), Some(log), None)
         .context("Failed to create VM")?;