Tweak handling of guest machine execution mode

[tkchia]

• Split machine mode m->mode into two parts, to explicitly distinguish real mode & 16-bit protected mode
  • m->mode.omode now says how instructions should be decoded
  • m->mode.genmode says whether guest is in real mode or not
• Use Mode(rde) more often, so we can optimize away dead code on --disable-metal
• Setting CR0_PE in guest %cr0 now immediately triggers machine mode change

[jart]

Thank you!