Introduce accessor generator macros for VCPU registers. Use the accessor
functions to replace direct accesses to this registers.

This will be important later for Nested APIv2 support which requires
additional functionality for accessing and modifying VCPU state.
Signed-off-by: Jordan Niethe <jniethe5@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20230914030600.16993-5-jniethe5@gmail.com