frontend: Fixed default color depth for spice-activex

This patch fixes setting default value of ColorDepth attribute of spice browser
plugin. For spice-xpi the value is "", whereas for spice-activex the value
is 0.

Change-Id: I58af05c0a2ff88dccb4634ec9a81dff84afe92a5
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=913183
Signed-off-by: Frantisek Kobzik <fkobzik@redhat.com>