CA-138822: Improvements to the Host/Pool Storage tab, following on from ...

...the TRIM work

- Make "Reclaim freed space" a CommandButton (will appear as disabled if the command cannot execute)
- Add tooltip to "Reclaim freed space" button to show the reason it is disabled
- Hide "Reclaim freed space" button on old servers
- Allow multiselect
- "Reclaim freed space" button enabled if at least one SR can be trimmed

Signed-off-by: Mihaela Stoica <mihaela.stoica@citrix.com>