devices/loopback: make setting sector_size meaningful

For the org.osbuild.loopback the user can set the sector size, but
it had no effect on the underlying loopback device. Let's make it
meaningful by passing along the given value to the underlying code.

devices/org.osbuild.loopback

@@ -82,6 +82,7 @@ class LoopbackService(devices.DeviceService):
                                   setup=self.setup_loop,
                                   offset=offset,
                                   sizelimit=sizelimit,
+                                  blocksize=self.sector_size,
                                   partscan=False,
                                   autoclear=True)