trivial: Do not set up the backends if there is no coldplug
trivial: Do not set up the backends if there is no coldplug

This speeds up fu-self-test, but more importantly fixes a intermittent
'ninja check' failure when a host USB device disconnects at exactly the
wrong time.

The FuUsbBackend code is unusual in that the setup() code sets up a
context (with a thread) which takes up to 2 seconds to timeout.

fwupd/fwupd,fwupd/fwupd,fwupd/fwupd,fwupd/fwupd,hughsie/fwupd,hughsie/fwupd,hughsie/fwupd,hughsie/fwupd