Implement SR.stat, returning global SR (ie filesystem) statistics

We return the total size of the filesystem and the amount of space
currently free. Note these values may change at any time, depending on
the filesystem implementation.

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
Update to 0.9.11

Signed-off-by: David Scott <dave.scott@eu.citrix.com>