xenstore: add xenstore-watch command line client

xenstore: add xenstore-watch command line client

# xenstore-watch x | while read w ; do
> echo "watch fired on $w"
> echo "value" $(xenstore-read $w)
> echo
> done

# xenstore-write x/y/z 42

output from while loop:
        watch fired on x/t/z
        value 42

[ also add line to .hgignore - iwj ]

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>