Import the Xen grant-table bus_dma(9) handlers from OpenBSD

Xen uses the grant table interface to grant foreign domains access to page frames and to do ownership transfer. Each domain maintains its own grant table and it shares that with Xen to tell it what permissions other domains have on its pages. The index of a grant table entry, called a grant reference, is transmitted to the other domains that need to access the page. OpenBSD’s implementation uses wrappers for the bus_dma(9) interface to implement custom bus_dma functions for communication between domains. This project will bring in the grant table handlers from OpenBSD and integrate them with the rest of the system.