Motivation for this change

When using LVM pool size field in libvirt_volume resource will be ignored because snapshot of base image is COW and cannot be resized. This patch solves this by adding extra parameter base_volume_copy, for more information see: dmacvicar/terraform-provider-libvirt#675

Part of working example (ubuntu container on nixos with libvirt):

 resource "libvirt_volume" "ubuntu_focal_image" {
  name = "ubuntu_focal_image"
  pool = "default"
  source = "/var/lib/libvirt/focal-server-cloudimg-amd64-disk-kvm.raw"
}

resource "libvirt_volume" "disk" {
  name = "u1-root"
  base_volume_id = libvirt_volume.ubuntu_focal_image.id
  base_volume_copy = true
  size = 100 * 1024 * 1024 * 1024
}

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.