Add a new instance query flag ‘disk_usage’

This patch adds a new instance query flag called disk_usage that
retrieves the overall space used by an instance on each of its nodes.
This can be used when balancing the cluster or checking N+1 status.

The flag is also exported in RAPI. Note the flag is currently broken for
file-based instances, as it represents the amount of space in the
cluster volume group.

Reviewed-by: ultrotter