xen-backend: Remove FIXME comment about user_creatable flag

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 <[email protected]>,
Cc: Peter Maydell <[email protected]>,
Cc: Thomas Huth <[email protected]>
Cc: [email protected]
Cc: Markus Armbruster <[email protected]>,
Cc: Marcel Apfelbaum <[email protected]>,
Cc: Laszlo Ersek <[email protected]>
Acked-by: Juergen Gross <[email protected]>
Acked-by: Marcel Apfelbaum <[email protected]>
Signed-off-by: Eduardo Habkost <[email protected]>
Message-Id: <20170503203604[email protected]>
Signed-off-by: Eduardo Habkost <[email protected]>