storage: optional 'refresh' elemement on pool

storage: optional 'refresh' elemement on pool

The new 'refresh' element can override the default refresh operations
for a storage pool. The only currently supported override is to set
the volume allocation size to the volume capacity. This can be specified
by adding the following snippet:

<pool>
...
  <refresh>
    <volume allocation='capacity'/>
  </refresh>
...
</pool>

This is useful for certain backends where computing the actual allocation
of a volume might be an expensive operation.

Signed-off-by: Jason Dillaman <dillaman@redhat.com>
Signed-off-by: Michal Privoznik <mprivozn@redhat.com>