qemu: fix error message with migrate2 xml

Otherwise, an ABI mismatch gives error messages attributing the target
xml string as current, and the current domain state as the new xml.

* src/qemu/qemu_migration.c (qemuMigrationBegin): Use correct
argument order.

src/qemu/qemu_migration.c

@@ -1037,7 +1037,7 @@ char *qemuMigrationBegin(struct qemud_driver *driver,
                                             VIR_DOMAIN_XML_INACTIVE)))
             goto cleanup;
 
-        if (!virDomainDefCheckABIStability(def, vm->def))
+        if (!virDomainDefCheckABIStability(vm->def, def))
             goto cleanup;
 
         rv = qemuDomainDefFormatXML(driver, def,