webadmin: custom preview snapshot dialog

VMs -> Snapshot sub-tab:
* Create snapshot dialog now includes disks list
  for customization.
* Replaced "Preview" simple action button with a
  "Preview -> Custom..." drop-down button.
* Added "Custom Preview" snapshot dialog for enabling
  'mix-and-match' of snapshots' components.

Change-Id: I58b37bdfb78c79306c6e7fde1e1d7f0c09b231ad
Feature: http://www.ovirt.org/Features/Single_Disk_Snapshot
Signed-off-by: Daniel Erez <derez@redhat.com>