storage: fix the error message when encrypted raw volume resize

The vol-dumpxml shows the volume target format type as raw for
encrypted volumes. The error message when attempting to resize
with prealloc is confusing here.
Signed-off-by: NShivaprasad G Bhat <sbhat@linux.vnet.ibm.com>
Reviewed-by: NJohn Ferlan <jferlan@redhat.com>

src/storage/storage_util.c

@@ -2343,8 +2343,8 @@ virStorageBackendVolResizeLocal(virStoragePoolObjPtr pool,
     } else if (vol->target.format == VIR_STORAGE_FILE_RAW && vol->target.encryption) {
         if (pre_allocate) {
             virReportError(VIR_ERR_OPERATION_UNSUPPORTED, "%s",
-                           _("preallocate is only supported for raw "
-                             "type volume"));
+                           _("preallocate is only supported for an "
+                             "unencrypted raw volume"));
             return -1;
         }