webadmin: New disk dialog does not validate the size value

The size field is defined as as an entity model of type long and so the
integer validator does not validate it at all making it possible for numbers
less than the minimum (including negative number and zero) as well as
numbers higher than the maximum to appear as validated, this was fixed
by replacing the entity model for size from type long to integer

Change-Id: Ie8f6e52bc11b248fd7829261df7ec3626603e363
Signed-off-by: Tal Nisan <tnisan@redhat.com>
Bug-Url: https://bugzilla.redhat.com/1115471