Add per-SR datasource URIs
Add per-SR datasource URIs

These should point to xenostat-format shared memory pages containing
time-varying data. Suitable uses include

- free space in the SR
- I/O latency

Signed-off-by: David Scott <63c9eb0ea83039690fefa11afe17873ba8278a56@citrix.com>

xapi-project/message-switch,xapi-project/message-switch,xapi-project/message-switch