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>

Cargo.toml

@@ -55,6 +55,7 @@ common = ["acpi", "cmos", "fwdebug"]
 acpi = ["vmm/acpi"]
 cmos = ["vmm/cmos"]
 fwdebug = ["vmm/fwdebug"]
+gdb = ["vmm/gdb"]
 kvm = ["vmm/kvm"]
 mshv = ["vmm/mshv"]
 tdx = ["vmm/tdx"]