Volume loses its format_type when saved

[mxey]

When creating a volume with fog-libvirt, the format_type is lost:

irb(main):004:0> compute = Fog::Compute.new(provider: :libvirt, libvirt_uri: "qemu+ssh://s098209.rz.babiel.com/system")
=> #<Fog::Libvirt::Compute::Real:5400 @uri=#<Fog::Libvirt::Util::URI:0x0000000109fd8068 @parsed_uri=#<URI::Generic qemu+ssh://s098209.rz....
irb(main):005:0> vol = compute.volumes.new
=> 
  <Fog::Libvirt::Compute::Volume
...

irb(main):006:0> vol.format_type
=> "raw"


irb(main):007:0> vol.save
=> 
  <Fog::Libvirt::Compute::Volume
    id="tqVRqA-UU7o-qOms-HaHd-e5hr-a4qi-Fz4hU7",
    pool_name="vg1",
    key="tqVRqA-UU7o-qOms-HaHd-e5hr-a4qi-Fz4hU7",
    name="fog-817509623589251",
    path="/dev/vg1/fog-817509623589251",
    capacity=10,
    allocation=1,
    owner=nil,
    group=nil,
    format_type=nil,
    backing_volume=nil
  >

irb(main):008:0> vol.format_type
=> nil

If this created volume is then used for a VM, the generated XML is invalid:

<driver name="qemu" type=""/>

which leads to the error Call to virDomainDefineXML failed: unsupported configuration: unknown driver format value ''

This causes https://projects.theforeman.org/issues/38081

[mxey]

This bug was introduced by 0277e9b - if I revert this, I cannot reproduce it anymore.