spice: add config options for the listening address

Make listening address configurable.  Also add options to
force using IPv4 or IPv6.
diff --git a/qemu-options.hx b/qemu-options.hx
index 5a0d26b..0410b90 100644
--- a/qemu-options.hx
+++ b/qemu-options.hx
@@ -682,6 +682,13 @@
 @item port=<nr>
 Set the TCP port spice is listening on for plaintext channels.
 
+@item addr=<addr>
+Set the IP address spice is listening on.  Default is any address.
+
+@item ipv4
+@item ipv6
+Force using the specified IP version.
+
 @item password=<secret>
 Set the password you need to authenticate.