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>

hw/xen/xen_backend.c

@@ -619,10 +619,7 @@ static void xendev_class_init(ObjectClass *klass, void *data)
 
     dc->props = xendev_properties;
     set_bit(DEVICE_CATEGORY_MISC, dc->categories);
-    /*
-     * FIXME: Set only because we are not sure yet if this device
-     * will be outside the q35 sysbus whitelist.
-     */
+    /* xen-backend devices can be plugged/unplugged dynamically */
     dc->user_creatable = true;
 }