The generic loader device is completely optional. Let's add a proper
config switch for it so that people can disable it if they don't need
it and want to create a minimalistic QEMU binary.

Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20190817101931.28386-9-thuth@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>