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.