MFC r303352

MFC r303352
    - Change the fbuf "vga" parameter to "vga=on|io|off".
       "io" is the default, and allows VGA i/o registers to be
       accessed. This is required by Win7/2k8 graphics guests that
       use a combination of BIOS int10 and UEFI.
       "off" disables all VGA i/o and mem accesses.
       "on" is not yet hooked up, but will enable full VGA rendering.

       OpenBSD/UEFI >= 5.9 graphics guests can be booted using "vga=off"

    - Allow "rfb" to be used instead of "tcp" for the fbuf VNC
      description. "tcp" will be removed at a future point and is
      kept as an alias.