Add the front end support for SPICE

This patch add the front end support of spice.

1 If there were a spice vm in host, show a "spice" button
but not "vnc" in guest page.

2 click "spice" we can show a screen just like the demo
http://www.spice-space.org/spice-html5/spice.html

Signed-off-by: zhoumeina <zhoumein@linux.vnet.ibm.com>
Modify vnc_proxy_port to display_proxy_port

Becasue vnc and spice are using the same port, and get the port from
back-end. So this var should be changed to display_proxy_port so that
it can be used by vnc and spice.

Signed-off-by: zhoumeina <zhoumein@linux.vnet.ibm.com>