xen: add xenstored user-space device

This device is used by the user-space daemon that runs xenstore
(xenstored). It allows xenstored to map the xenstore memory page, and
reports the event channel xenstore is using.

Sponsored by: Citrix Systems R&D

dev/xen/xenstore/xenstored_dev.c:

• Add the xenstored character device that's used to map the xenstore memory into user-space, and to report the event channel used by xenstore.

conf/files:

• Add the device to the build process.