Providers can support multiple box formats if they choose to.

Currently, providers must match a box format exactly the same
as that provider's name. i.e. the virtuabox provider needs a
"virtualbox" box and the "vmware_fusion" provider needs a
"vmware_fusion" box. Now, the provider can specify what the box format
is they want and support multiple if wanted.

Other box formats are specified in the provider definition within
a plugin:

    class Plugin < Vagrant.plugin("2", "provider")
      # ... other stuff

      provider("foo", box_format: ["virtualbox", "other_format"]) do
        # .. same
      end
    end

Now when using the example "foo" provider above, boxes for both
"virtualbox" or "other_format" are searched for. If both are found,
the order in which the formats exist determines precedence.