Import bhyve_graphics into CURRENT. Thanks to all who tested

this on the branch.

Original commit message:
  Initial bhyve native graphics support.

  This adds emulations for a raw framebuffer device, PS2 keyboard/mouse,
  XHCI USB controller and a USB tablet.

  A simple VNC server is provided for keyboard/mouse input, and graphics
  output.

  A VGA emulation is included, but is currently disconnected until an
  additional bhyve change to block out VGA memory is committed.

  Credits:
   - raw framebuffer, VNC server, XHCI controller, USB bus/device emulation
      and UEFI f/w support by Leon Dang
   - VGA, console/g, initial VNC server  by tychon@
   - PS2 keyboard/mouse jointly done by tychon@ and Leon Dang
   - hypervisor framebuffer mem support by neel@

  Tested by: Michael Dexter, in a number of revisions of this code.

  With the appropriate UEFI image, FreeBSD, Windows and Linux guests can
  installed and run in graphics mode using the UEFI/GOP framebuffer.

Approved by:	re (gjb)