virstoragetest: Add test case for NBD over unix socket with new syntax

Use the new syntax which uses the 'UnixSocket' type in qemu.
Signed-off-by: NPeter Krempa <pkrempa@redhat.com>

tests/virstoragetest.c

@@ -1494,6 +1494,15 @@ mymain(void)
                        "<source protocol='nbd' name='blah'>\n"
                        "  <host name='example.org' port='6000'/>\n"
                        "</source>\n");
+    TEST_BACKING_PARSE("json:{\"file\":{\"driver\":\"nbd\","
+                                       "\"server\": { \"type\":\"unix\","
+                                                     "\"path\":\"/path/socket\""
+                                                   "}"
+                                      "}"
+                            "}",
+                       "<source protocol='nbd'>\n"
+                       "  <host transport='unix' socket='/path/socket'/>\n"
+                       "</source>\n");
     TEST_BACKING_PARSE("json:{\"file\":{\"driver\":\"ssh\","
                                        "\"host\":\"example.org\","
                                        "\"port\":\"6000\","