frontend: fixed layout in console options dialog

This patch fixes wrong float of console protocol specific options block.
It also adds 'Console Invocation' label to the RDP section to be
consistent with SPICE and VNC section.

Change-Id: Ib390d4bfb638d7f50bc3aa041b143460b9c2f6bd
Signed-off-by: Frantisek Kobzik <fkobzik@redhat.com>