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.