webadmin: Show only supported displays

This change uses the information from osinfo to avoid the selection of
SPICE on operating systems that do not support it. This is also used to
disable the SPICE protocol on POWER hosts, because the operating
systems are architecture specific.

Change-Id: I5c91524181a7fa1fc2961c85df9bb78f553aa1ef
Signed-off-by: Vitor de Lima <vitor.lima@eldorado.org.br>
Signed-off-by: Gustavo Pedrosa <gustavo.pedrosa@eldorado.org.br>