honour path parameter of storage directive

Fix the following:

    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.