Along with video and VNC support, bhyve has introduced USB tablet
support as an input device. This tablet is exposed to a guest
as a device on an XHCI controller.

At present, tablet is the only supported device on the XHCI controller
in bhyve, so to make things simple, it's allowed to only have a
single XHCI controller with a single tablet device.

In detail, this commit:

 - Introduces a new capability bit for XHCI support in bhyve
 - Adds an XHCI controller and tabled support with 1:1 mapping
   between them
 - Adds a couple of unit tests