frontend: "escape" underscores in spice menu

this patch doubles underscores in spice menu since single underscore is
interpreted as a hotkey by gtk and is not printed (i.e. fedora_20.iso
was printed as fedora20.iso).

Change-Id: Ibb090a6ae37aad234bf815c40b8dd5a051b24879
Bug-Url: https://bugzilla.redhat.com/963307
Signed-off-by: Frantisek Kobzik <fkobzik@redhat.com>