virt: Add a func to get graphic devices

Signed-off-by: Li Yang <liyang.fnst@cn.fujitsu.com>
virt: Modify add_ssl_spice_graphic to make it can add vnc type

Signed-off-by: Li Yang <liyang.fnst@cn.fujitsu.com>
Merge pull request #1758 from liyangfnst/vnc

Add/Modify functions for graphic device