xenstore: add support for reading directory with many children

xenstore: add support for reading directory with many children

As the payload size for one xenstore wire command is limited to 4096
bytes it is impossible to read the children names of a node with a
large number of children (e.g. /local/domain in case of a host with
more than about 2000 domains). This effectively limits the maximum
number of domains a host can support.

In order to support such long directory outputs add a new wire command
XS_DIRECTORY_PART which will return only some entries in each call and
can be called in a loop to get all entries.

Input data are the path of the node and the byte offset into the child
list where returned data should start.

Output is the generation count of the node (which will change each time
the node is being modified) and a list of child names starting with
the specified index. The end of the list is indicated by an empty
child name. It is the responsibility of the caller to check for data
consistency by comparing the generation counts of all returned data
sets to be the same for one node.

Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Wei Liu <wei.liu2@citrix.com>