read-only register access for plugins: - move misa_mxl to CPU class for riscv - use GDBFeature for arm XML - use GDBFeature for ppc XML - use GDBFeature for riscv XML - unify gdb code to use GDBFeature - move dynamic XML generation to core GDB code - provide introspection APIs for rest of QEMU - expose a plugin API to access registers - fix memory re-use in execlog - extend execlog to track registers - optimise instrumentation based on disassembly - tweak API docs and expand on assumptions