chardev/spice: simplify chardev setup

Initialize spice before chardevs.  That allows to register the spice
chardevs directly in the init function and removes the need to maintain
a linked list of chardevs just for registration.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Message-id: 20201014121120.13482-5-kraxel@redhat.com