Let data-size be specified as -1

Previously, we enforced bounds of >= 0 on data size, but -1 should
be an allowed size to fill the entire partition. Allow it.

Change-Id: Idef165f508bd7628311c04edc240f3af006d7577
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1017583
Signed-off-by: Ryan Barry <rbarry@redhat.com>