spice: introduce listen type none

spice: introduce listen type none

This new listen type is currently supported only by spice graphics.
It's introduced to make it easier and clearer specify to not listen
anywhere in order to start a guest with OpenGL support.

The old way to do this was set spice graphics autoport='no' and don't
specify any ports.  The new way is to use <listen type='none'/>.  In
order to be able to migrate to old libvirt the migratable XML will be
generated without the listen element and with autoport='no'.  Also the
old configuration will be automatically converted to the this listen
type.

Signed-off-by: Pavel Hrdina <phrdina@redhat.com>