dev/xenstore: add support for watches

Allow user-space applications to register watches using the xenstore
device. This is needed in order to run toolstack operations on
domains different than the one where xenstore is running (in which
case the device is not used, since the connection to xenstore is done
using a plain socket).

Tested by: Nathan Friess <nathan.friess@gmail.com>
Sponsored by: Citrix Systems R&D