xen: initialize legacy backends from xen_bus_init()

xen: initialize legacy backends from xen_bus_init()

Prepare for moving the calls to xen_be_register() under the
control of xen_bus_init(), using the normal xen_backend_init()
method that is used by the "modern" backends.

This requires the xenstore global variable to be initialized,
which is done by xen_be_init().  To ensure that everything is
ready at the time the xen_backend_init() functions are called,
remove the xen_be_init() function from all the boards and
place it directly in xen_bus_init().

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-ID: <20240509170044.190795-7-pbonzini@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>