Allow TCG plugins to read registers
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