xen: move grant table code

Mave the grant table code into the dev/xen folder in preparation for turning
it into a device using the newbus interface. This is just code motion, no
functional changes.

Sponsored by: Citrix Systems R&D
o Put () for p to avoid unexpected macro expansion
o style(9) fix

Submitted by:	Yonghyeon PYUN <pyunyh@gmail.com>
xen: convert the Grant-table code to a NewBus device

This allows the Grant-table code to attach directly to the xenpv bus,
allowing us to remove the grant-table initialization done in xenpv.

Sponsored by: Citrix Systems R&D