x86/vtx: Introduce a typed union for CR access exit information

x86/vtx: Introduce a typed union for CR access exit information

This reduces code volume, and has a minor improvement on compiled size,
probably due to the removal of several temporary variables.

  add/remove: 0/0 grow/shrink: 0/2 up/down: 0/-50 (-50)
  function                                     old     new   delta
  vmx_vmexit_handler                          6881    6878      -3
  nvmx_n2_vmexit_handler                      3473    3426     -47

Take the opportunity to make some style corrections, and add some
ASSERT_UNREACHABLE()s in appropriate places.

No functional change.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Kevin Tian <kevin.tian@intel.com>