UI Provision code to filter Amazon instance types

https://bugzilla.redhat.com/show_bug.cgi?id=1133780

The instance types are filtered based on the virtualization
type specified in the AMI. Each instance can support 1 or more
kind(s) of virtualization: hvm, paravirtual. Each AMI has a
virtualization type, so based on the selected AMI we can whittle
down the list of applicable instance types.
Again can you make the code consistent with architecture...the two bits of code should visually look almost identical.  I'd almost go so far as saying we should extract a method, but it's not necessary.  I think the code should look like

```ruby
return {} if ems.nil?
method = "supports_#{architecture}_bit?"
available = ems.flavors.select { |f| f.respond_to?(method) && f.send(method) }
method = "supports_#{virtualization_type}?"
available = available.select { |f| f.respond_to?(method) && f.send(method) }
```

Or even nicer would be...

```ruby
return {} if ems.nil?
method = "supports_#{architecture}_bit?".to_sym
available = ems.flavors.select(&method) if FlavorAmazon.method_defined?(method)
method = "supports_#{virtualization_type}?".to_sym
available = available.select(&method) if FlavorAmazon.method_defined?(method)
```