xen: register legacy backends via xen_backend_init

xen: register legacy backends via xen_backend_init

It is okay to register legacy backends in the middle of xen_bus_init().
All that the registration does is record the existence of the backend
in xenstore.

This makes it possible to remove them from the build without introducing
undefined symbols in xen_be_init().  It also removes the need for the
backend_register callback, whose only purpose is to avoid registering
nonfunctional backends.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-ID: <20240509170044.190795-8-pbonzini@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>