pvusb: don't rely on linux kernel macros for the interface

pvusb: don't rely on linux kernel macros for the interface

The interface description of pvUSB lacks some access macros as using
linux kernel macros is assumed to work well. This solution is rather
unfriendly for pvusb implementations being outside the linux kernel.
Additionally things will break quite unpleasent in case the linux
kernel implementation is changed.

To avoid these problems define own macros for accessing bitfields of
the interface and for values of several structure members.

While working on the file add some more comments, especially for the
xenstore interface.

Signed-off-by: Juergen Gross <jgross@suse.com>