Comment 11 for bug 1847105

## Description of problem: ##
Hi,
in Ubuntu bug 1847105 [1] I think I have found a somewhat unexpected or unclear behavior mismatch between libvirt `>v4.3` and virt-manager.

For volume creation virt-manager since quite some time (I checked back until `v1.5`) will submit an XML like the following to libvirts volume creation:

<volume>
  <name>ubuntu19.04.qcow2</name>
  <capacity>16106127360</capacity>
  <allocation>16106127360</allocation>
  <target>
    <format type="qcow2"/>
    <features>
      <lazy_refcounts/>
    </features>
  </target>
</volume>

The important detail in regard to this issues is that capacity==allocation.
In the definition by libvirt [2] this is somewhat undefined. In my reading of it this case would also fully allocate the volume.

But - and here is the change in behavior - up until libvirt 'v4.3' it didn't.
It used to use `preallocation=metadata` until a change [3] made it issue `preallocation=falloc` nowadays.

eventual qemu-img call:
Old: `/usr/bin/qemu-img create -f qcow2 -o preallocation=metadata,compat=1.1,lazy_refcounts /var/lib/libvirt/images/ubuntu18.04.qcow2 15728640K`
New: `/usr/bin/qemu-img create -f qcow2 -o preallocation=falloc,compat=1.1,lazy_refcounts /var/lib/libvirt/images/live-server.qcow2 41943040K`

## Version-Release number of selected component (if applicable): ##
v1.5 - v2.2.1

## How reproducible: ##
100%

## Steps to Reproduce: ##
1. create a new guest (use PXE to not require any ISO)
2. when you hit "Finish" to start the installation it will issue a qemu-img command
3. with newer libvirt versions this will use preallocation=falloc

## Actual results: ##
qemu-img ... preallocation=falloc

## Expected results: ##
qemu-img ... preallocation=metadata (like in the past)
OR a statement that falloc is preferred by virt-manager as well and not just an accident due to a change in libvirt.

## Next steps: ##
I was now wondering about virt-managers position on this, do you:
 * want to modify virt-manager to get back to spare/lazy allocations
   (TL;DR submit an allocation value that is `0<allocation<capacity`?
 * consider this change a bug in libvirt, so that we should report/change
   it there? Maybe the comparison there should be `>=` and not just `>`
   in your opinion?
 * not changing anything as the `falloc` creation is preferred from the
   POV of virt-manager
 * some other way answer I didn't predict :-)

[1]: https://bugs.launchpad.net/ubuntu/+source/virt-manager/+bug/1847105
[2]: https://libvirt.org/formatstorage.html#StorageVol
[3]: https://libvirt.org/git/?p=libvirt.git;a=blobdiff;f=src/storage/storage_util.c;h=897dfdaaee4da967fb5dacc4327484f40b8aa717;hp=b4aed0f7009b8d7e81a64e4f4a5c920a422fbba4;hb=c9ec7088c7a3f4cd26bb471f1f243931fff6f4f9;hpb=c1bc9c662b411f8fa06071991921a180480a0f40