add low level functions from the root (read/write). [PATCH 3/3]

add low level functions from the root (read/write).  [PATCH 3/3]

This is a series of patch to complete the xenstore interface in qemu.

1: create a callback mecanism for xenstore watches.
2: add per domain low level xenstore functions (read/chmod/write/watch).
3: add low level functions from the root (read/write).

Signed-off-by: Jean Guyader <jean.guyader@citrix.com>