core: Fix NPE when disk size is not yet set

Available disk size is set to null on Storage Domain creation, to be
later polled and updated. This patch protects from the period of time
this value is null, a situation that can occur in storage allocation
validations on new domains.

Change-Id: Icca933dcf506ca702161fcfbfe400a7259d20d1a
Bug-Url: https://bugzilla.redhat.com/1176854
Signed-off-by: Vered Volansky <vvolansk@redhat.com>