spice: fix "info spice"

In case no listening address was specified, "info spice" reports
"0.0.0.0" as address.  Which is incorrect in case spice is listening
on ipv6.  Replace it by a wildcard "*" to indicate it is not limited
to a specific address.

Note:  Being more specific is not possible without extending the
spice-server api.  The socket is handled by spice-server not
qemu, so qemu can't easily figure the actual socket address.

Reported-by: David Jaša <djasa@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
