Switch over all leftover users to qapi DisplayType.
Then delete the unused display_type variable.

Add 'default' DisplayType, which isn't an actual display type but
a placeholder for "user didn't specify a display".  It will be replaced
by the DisplayType actually used, which in turn depends on the
DisplayTypes availabel in the particular build.
Signed-off-by: NGerd Hoffmann <kraxel@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
Message-id: 20180202111022.19269-13-kraxel@redhat.com
Signed-off-by: NGerd Hoffmann <kraxel@redhat.com>