convert disk list to disk tree and conditionally include partitions

convert disk list to disk tree and conditionally include partitions

Assigning the store directly to the treepanel doesn't work, more manual
handling is needed. This is mostly based on what we do for PBS's datastore
content view. The store monitoring also needs to be changed slightly.

The buttons are restricted to work on disks only, based on the parent
attribute, that only partitions have.

Signed-off-by: Fabian Ebner <f.ebner@proxmox.com>