Fix storage volume page

Don't check for the presence of values in the storage volume page
if _model is empty. Don't directly use ints in the model. Try to
make the model in dry mode resemble the model in a normal
install.

Change-Id: I716a9ea86f52ab3282c4be55f8dcad0890df0583
Signed-off-by: Ryan Barry <rbarry@redhat.com>