libvirt.test: Modify add_ssl_spice_graphic func to add_graphic

Signed-off-by: Li Yang <liyang.fnst@cn.fujitsu.com>
libvirt.test: Fix hidden bug in domdisplay test

Befor test, check if domain has graphic device, if
no graphic device exists, add one device.

Signed-off-by: Li Yang <liyang.fnst@cn.fujitsu.com>
libvirt.test: Fix hidden failure in test

If domain doesn't have a vnc graphic device, all the
normal tests will be fail. Now before execute vncdisplay
command, test will check if vnc graphic device exists,
if not, test will add a vnc graphic device to domain.

Signed-off-by: Li Yang <liyang.fnst@cn.fujitsu.com>