storage: allow zero capacity with non-backing file to be created

In commit 155ca616, a change was introduced that no longer allowed defining
volumes via XML with a capacity of '0'. Because we check for info.size_arg
to be non-zero, this use-case fails. This patch allows info.size_arg to be
zero if no backing store is specified.
Signed-off-by: NChris J Arges <chris.j.arges@canonical.com>

src/storage/storage_backend.c

@@ -1075,7 +1075,7 @@ virStorageBackendCreateQemuImgCmdFromVol(virConnectPtr conn,
     if (info.inputPath)
         virCommandAddArg(cmd, info.inputPath);
     virCommandAddArg(cmd, info.path);
-    if (!info.inputPath && info.size_arg)
+    if (!info.inputPath && (info.size_arg || !info.backingPath))
         virCommandAddArgFormat(cmd, "%lluK", info.size_arg);
 
     return cmd;