gui: qemu: HardwareView: improve button disable code

gui: qemu: HardwareView: improve button disable code

this improves following behaviours:
* do not loop over items multiple times
  (we iterated one time extra for efidisk)
* do not check every item for cloudinit
  (we even checked non-disk types)
* make the counts more generic using an object
* also disable cloudinit button when you have one pending
* also disable serial/net button when at limit or the user has
  not the right priviliges

Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>