Fix mkfs failure when loop device is not ready

There was a race in diskimage-builder where the mkfs call after a
kpartx -avs for the loop device would fail because the device was
not yet ready.  This adds a udevadm settle call after the kpartx
to make sure the udev event queue has cleared.

Change-Id: I90103b59357edebbac7a641e8980cb282d37561b
Closes-Bug: #1698337

diskimage_builder/block_device/level1/partitioning.py

@@ -147,6 +147,12 @@ class Partitioning(PluginBase):
         # mount them
         if not os.path.exists("/.dockerenv"):
             exec_sudo(["kpartx", "-avs", device_path])
+            # We need to make sure udev finishes creating the device
+            # before continuting, so "udevadm settle".  Otherwise later
+            # commands can fail with "file does not exist".
+            # XXX: "-s" (synchronous) to kpartx should avoid this,
+            # but experience shows it does not.
+            exec_sudo(["udevadm", "settle"])
         else:
             # If running inside Docker, make our nodes manually,
             # because udev will not be working. kpartx cannot run in