storage: fix logical volume cloning

Commit 258e06c8 removed setting of the volume type to
VIR_STORAGE_VOL_BLOCK, which leads to failures in
storageVolumeCreateXMLFrom.

The type (and target.format) of the volume was set to zero. In
virStorageBackendGetBuildVolFromFunction, this gets interpreted as
VIR_STORAGE_FILE_NONE and the qemu-img tool is called with unknown
"none" format.

Bug: https://bugzilla.redhat.com/show_bug.cgi?id=879780
(cherry picked from commit 70f0bbe8)

src/storage/storage_backend_logical.c

@@ -708,6 +708,8 @@ virStorageBackendLogicalCreateVol(virConnectPtr conn,
         return -1;
     }
 
+    vol->type = VIR_STORAGE_VOL_BLOCK;
+
     if (vol->target.path != NULL) {
         /* A target path passed to CreateVol has no meaning */
         VIR_FREE(vol->target.path);