webadmin: custom snapshot preview - default disks selection

Custom snapshot dialog:
if a partial snapshot has been pre-selected, the missing disks
should be selected on the active snapshot by default.

Change-Id: I78c1c11802c72c8eef91a25905078ce309dd03ea
Bug-Url: https://bugzilla.redhat.com/1071791
Signed-off-by: Daniel Erez <derez@redhat.com>