xen-backend: Remove FIXME comment about user_creatable flag

xen-backend can be plugged/unplugged dynamically when using the
Xen accelerator, so keep the user_creatable flag on the device
class and remove the FIXME comment.

Cc: Juergen Gross <jgross@suse.com>,
Cc: Peter Maydell <peter.maydell@linaro.org>,
Cc: Thomas Huth <thuth@redhat.com>
Cc: sstabellini@kernel.org
Cc: Markus Armbruster <armbru@redhat.com>,
Cc: Marcel Apfelbaum <marcel@redhat.com>,
Cc: Laszlo Ersek <lersek@redhat.com>
Acked-by: Juergen Gross <jgross@suse.com>
Acked-by: Marcel Apfelbaum <marcel@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Message-Id: <20170503203604.31462-4-ehabkost@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>