honour path parameter of storage directive

Fix use of the `:path` parameter to the `storage` directive:

    config.vm.provider :libvirt do |libvirt|
      libvirt.storage :file, :path => 'my-disk.qcow2'
    end

The value of the `:path` parameter was being correctly used in `domain.xml.erb` for defining the VM, but it was not passed to libvirt via fog's volume creation mechanism:

    lib/fog/libvirt/models/compute/templates/volume.xml.erb

Instead, libvirt determines the backing file from the `<name>` element, so we ensure that the `:name` parameter contains the correct filename.