x86: Allow index-based guest register access without type casts

x86: Allow index-based guest register access without type casts

Convert struct registers into a union and provide a by_index array for
index-based access. This is used by various handlers that parse guest
instructions and so far use a blunt type case on the structure.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>