Add USB disk

For some of the tests, we need to create small disk that
we will connect to the openQA virtual machine as a USB disk.
This adds a simple disk with some content to act like a "real"
usb flash drive.

Fixes: https://pagure.io/fedora-qa/createhdds/issue/17

hdds.json

@@ -126,6 +126,25 @@
                     "content" : "Hello, world!"
                 }
             ]
+        },
+        {
+            "name" : "usb",
+            "size" : "4G",
+            "parts" : [
+                {
+                    "type" : "p",
+                    "filesystem" : "vfat",
+                    "start" : "4096",
+                    "end" : "-1"
+                }
+            ],
+            "writes" : [
+                {
+                    "part" : "1",
+                    "path" : "/testfile",
+                    "content" : "Hello, world!"
+                }
+            ]
         }
     ],
     "virtinstall" : [