Allow TCG plugins to read registers

Goal

Access particular register values from the TCG plugin

Technical details

Currently there is QEMU_PLUGIN_CB_R_REGS enum value but it's never used anywhere. Adding a hook that has the access to register values is valuable in many different scenarios.

Additional information

• include/qemu/plugin.h
• include/qemu/qemu-plugin.h

PANDA implemented this already but it is not a very clean solution:

• https://github.com/panda-re/qemu/commit/b97c5a56edd0ba3b5f6ab16bf531ac1f7abaac04 (mentioned in QPP patch series: https://lore.kernel.org/qemu-devel/20221213213757.4123265-1-fasano@mit.edu/)

I personally think the flag for the TB translation and execution callbacks makes more sense