phyp: fix logic error on volume creation

phyp: fix logic error on volume creation

The phyp code claims that it wants a non-zero value, but actually
enforces a capacity of zero.  It has been this way since commit
ebc46fe in June 2010.  Bummer that it has my name as the committer
- I guess I should have been much more stubborn about not blindly
taking someone else's 1600-line patch.

* src/phyp/phyp_driver.c (phypStorageVolCreateXML): Use correct
logic.

Signed-off-by: Eric Blake <eblake@redhat.com>