webadmin: Fixed a bug in disks allocation when importing a VM

When importing a VM without collapse snapshots the allocation should fit the
original allocation and should not be changeable, when moving to collapse
snapshots the allocation should be changeable and keep the same logic when
switching from with or without collapse snapshot option

Change-Id: Iff56b64302b4f0a07a4c6bbcfb0cccdf9533e292
Bug-Url: https://bugzilla.redhat.com/965170
Signed-off-by: Tal Nisan <tnisan@redhat.com>