frontend: Spice proxy checkbox behavior for spice-html5

This patch disables and unchecks spice proxy checkbox in case user
selects spice-html5 console (spice proxy is not supported for this
console mode).

Change-Id: I213e6295d1356447120d7c6395189307634cd0b1
Bug-Url: https://bugzilla.redhat.com/1059349
Signed-off-by: Frantisek Kobzik <fkobzik@redhat.com>