webadmin: filter out unsupported architectures

Unsupported architecture is one that doesn't have a configured cpu to
support it (its a vdc_options). Those archs are useless effectively so
no point showing them in the dropdown.

Bug-Url: https://bugzilla.redhat.com/1081502
Change-Id: I7568e61882478f10bec614ddf514b32d23adc199
Signed-off-by: Roy Golan <rgolan@redhat.com>