xen-block: Avoid leaks on new error path

Commit 1898293 ("xen-block: Use specific blockdev driver")
introduced a new error path, without taking care of allocated
resources.

So only allocate the qdicts after the error check, and free both
`filename` and `driver` when we are about to return and thus taking
care of both success and error path.

Coverity only spotted the leak of qdicts (*_layer variables).

Reported-by: Peter Maydell <peter.maydell@linaro.org>
Fixes: Coverity CID 1508722, 1398649
Fixes: 1898293 ("xen-block: Use specific blockdev driver")
Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Reviewed-by: Paul Durrant <paul@xen.org>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Message-Id: <20230704171819.42564-1-anthony.perard@citrix.com>
Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>

hw/block/xen-block.c

@@ -781,22 +781,22 @@ static XenBlockDrive *xen_block_drive_create(const char *id,
     drive = g_new0(XenBlockDrive, 1);
     drive->id = g_strdup(id);
 
-    file_layer = qdict_new();
-    driver_layer = qdict_new();
-
     rc = stat(filename, &st);
     if (rc) {
         error_setg_errno(errp, errno, "Could not stat file '%s'", filename);
         goto done;
     }
+
+    file_layer = qdict_new();
+    driver_layer = qdict_new();
+
     if (S_ISBLK(st.st_mode)) {
         qdict_put_str(file_layer, "driver", "host_device");
     } else {
         qdict_put_str(file_layer, "driver", "file");
     }
 
     qdict_put_str(file_layer, "filename", filename);
-    g_free(filename);
 
     if (mode && *mode != 'w') {
         qdict_put_bool(file_layer, "read-only", true);
@@ -831,7 +831,6 @@ static XenBlockDrive *xen_block_drive_create(const char *id,
     qdict_put_str(file_layer, "locking", "off");
 
     qdict_put_str(driver_layer, "driver", driver);
-    g_free(driver);
 
     qdict_put(driver_layer, "file", file_layer);
 
@@ -842,6 +841,8 @@ static XenBlockDrive *xen_block_drive_create(const char *id,
     qobject_unref(driver_layer);
 
 done:
+    g_free(filename);
+    g_free(driver);
     if (*errp) {
         xen_block_drive_destroy(drive, NULL);
         return NULL;