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
ebc46fea 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: NEric Blake <eblake@redhat.com>

src/phyp/phyp_driver.c

@@ -2007,7 +2007,7 @@ phypStorageVolCreateXML(virStoragePoolPtr pool,
         goto err;
     }
 
-    if (voldef->capacity) {
+    if (!voldef->capacity) {
         VIR_ERROR(_("Capacity cannot be empty."));
         goto err;
     }