My previous commit, f92f8afe,  broke -vnc (spotted by Glauber Costa).  This
is because it's necessary to tell when the no special display parameters have
been passed and default to SDL or VNC appropriately.

This refactors the display selection logic to be less complicated which has
the effect of fixing the regression mentioned above.

Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>