tests: virstorage: Add tests for NBD URI style syntax over UNIX

Add few test cases for nbd+unix style URIs with few corner cases.

The NBD URI syntax is documented at
https://github.com/NetworkBlockDevice/nbd/blob/master/doc/uri.mdSigned-off-by: NPeter Krempa <pkrempa@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
Reviewed-by: NJán Tomko <jtomko@redhat.com>

tests/virstoragetest.c

@@ -1274,6 +1274,26 @@ mymain(void)
                        "<source protocol='nbd' name='exportname'>\n"
                        "  <host name='example.org' port='1234'/>\n"
                        "</source>\n");
+    TEST_BACKING_PARSE("nbd+unix://?socket=/tmp/sock",
+                       "<source protocol='nbd'>\n"
+                       "  <host transport='unix' socket='/tmp/sock'/>\n"
+                       "</source>\n");
+    TEST_BACKING_PARSE("nbd+unix:///?socket=/tmp/sock",
+                       "<source protocol='nbd'>\n"
+                       "  <host transport='unix' socket='/tmp/sock'/>\n"
+                       "</source>\n");
+    TEST_BACKING_PARSE("nbd+unix:////?socket=/tmp/sock",
+                       "<source protocol='nbd' name='/'>\n"
+                       "  <host transport='unix' socket='/tmp/sock'/>\n"
+                       "</source>\n");
+    TEST_BACKING_PARSE("nbd+unix:///exp?socket=/tmp/sock",
+                       "<source protocol='nbd' name='exp'>\n"
+                       "  <host transport='unix' socket='/tmp/sock'/>\n"
+                       "</source>\n");
+    TEST_BACKING_PARSE("nbd+unix:////exp?socket=/tmp/sock",
+                       "<source protocol='nbd' name='/exp'>\n"
+                       "  <host transport='unix' socket='/tmp/sock'/>\n"
+                       "</source>\n");
     TEST_BACKING_PARSE_FULL("iscsi://testuser:testpass@example.org:1234/exportname",
                             "<source protocol='iscsi' name='exportname'>\n"
                             "  <host name='example.org' port='1234'/>\n"