core: Use Collections instead of Lists

In storage allocation-related methods, use Collections instead if lists
when possible. Further validations will be on top of this, specifically
storage allocation validation in ImportVmTemplateCommand, hence the bz
url bellow.

Change-Id: Ic89752086a68ca2f87bc58cefc33d5373c7f8eef
Bug-Url: https://bugzilla.redhat.com/1136721
Signed-off-by: Vered Volansky <vvolansk@redhat.com>