Allow NULL mac address in virGetInterface

There are places where an interface will not have a mac address, and netcf
returns this as a NULL pointer rather than a pointer to an empty string.
Rather than checking for this all over the place in libvirt, just save it
in the virInterface object as an empty string.

* src/datatypes.c: allow NULL mac in virGetInterface()

src/datatypes.c

@@ -588,10 +588,15 @@ virInterfacePtr
 virGetInterface(virConnectPtr conn, const char *name, const char *mac) {
     virInterfacePtr ret = NULL;
 
-    if ((!VIR_IS_CONNECT(conn)) || (name == NULL) || (mac == NULL)) {
+    if ((!VIR_IS_CONNECT(conn)) || (name == NULL)) {
         virLibConnError(NULL, VIR_ERR_INVALID_ARG, __FUNCTION__);
         return(NULL);
     }
+
+    /* a NULL mac from caller is okay. Treat it as blank */
+    if (mac == NULL)
+       mac = "";
+
     virMutexLock(&conn->lock);
 
     ret = (virInterfacePtr) virHashLookup(conn->interfaces, name);