vmm: Add Debuggable trait implementation

This commit adds initial gdb.rs implementation for `Debuggable` trait to
describe a debuggable component. Some part of the trait bound
implementations is based on the crosvm GDB stub code [1].

[1] https://github.com/google/crosvm/blob/main/src/gdb.rs

Signed-off-by: Akira Moroo <retrage01@gmail.com>

vmm/src/lib.rs

@@ -57,6 +57,8 @@ pub mod config;
 pub mod cpu;
 pub mod device_manager;
 pub mod device_tree;
+#[cfg(feature = "gdb")]
+mod gdb;
 pub mod interrupt;
 pub mod memory_manager;
 pub mod migration;