engine: Change default display type for x86-64 VMs

After the change #20667, it is necessary put the default display
protocol in the first position in the 'display.protocols' list.

Change-Id: Iaba4996a482a0613282b45b19a658e8694a8d12b
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1057053
Signed-off-by: Gustavo Pedrosa <gustavo.pedrosa@eldorado.org.br>