For vagrant-libvirt, the second disk can be added w/ libvirt.storage :file, :size => '5G'.

To make use of this disk, we can use docker-storage-setup, as described in http://www.projectatomic.io/docs/docker-storage-recommendation.

The lines in the config.vm.provision section below add DEVS="/dev/vdb" to /etc/sysconfig/docker-storage-setup. The commented-out line is supposed to run docker-storage-setup to set up the storage, but this isn't working when run from the Vagrantfile here. Once logged in to the vagrant VM, however, running sudo docker-storage-setup works as expected, except for a syntax error with part of the setup script which doesn't appear to stop the script from doing its job.

Vagrant.configure("2") do |config|

  config.vm.box = "f22atomic"

  config.vm.provider "libvirt" do |libvirt|
    libvirt.driver = "kvm"
    libvirt.memory = 2048
    libvirt.cpus = 2
    libvirt.storage :file, :size => '5G'
  end

  config.vm.provision "shell" do |s|
    s.inline = "echo $1 > /etc/sysconfig/docker-storage-setup"
    s.args = ['DEVS="/dev/vdb"']
  end

  config.vm.provision "shell" do |s|
    s.inline = "docker-storage-setup"
  end

end

The syntax error I mentioned, issue upstream: projectatomic/container-storage-setup#5

failed [sfd_geom:1] sfdisk /dev/vda --show-pt-geometry
sfdisk: unrecognized option '--show-pt-geometry'

Usage:
 sfdisk [options] <dev> [[-N] <part>]
 sfdisk [options] <command>

Display or manipulate a disk partition table.

Commands:
 -a, --activate <dev> [<part> ...] list or set bootable MBR partitions
 -d, --dump <dev>                  dump partition table (usable for later input)
 -g, --show-geometry [<dev> ...]   list geometry of all or specified devices