Comment 5 for bug 1684239

Gah obviously for vvfat it isn't an image to be created at all.
So I thought steps to reproduce are:

1. get a guest that works (via uvtoools-libvirt or whatever you prever)
2. get the qemu commandline that it is started with, in my case:
   $ sudo kvm -m 1024 -drive file=/dev/sdb,format=raw,cache=none,index=0,media=disk
3. create a dir (can be empty) to share in vvfat mode
   $ mkdir /tmp/sharevvfat
   $ touch /tmp/sharevvfat/hostfoo
4. start your guest with the path as vvfat set to share (ro for now)
   Append the following to your qemu cmdline:
     -drive format=vvfat,file=fat:/tmp/sharevvfat/,if=virtio
   check the block device content by mounting it and verifyinf that "hostfoo" is there
5. shutdown the guest to retry rw
6. For rw append it as:
     -drive format=vvfat,file=fat:rw:/tmp/sharevvfat/,if=virtio

I have echoed some content into a file, and while the update policy isn't instand after my guest shut down I could access on the host what the guest put there.
All worked fine, but it seems it was already failing in the background on guest shutdown I see the failing assertion and the guest data is not written correctly to the Host.

Retrying on a newer qemu than Xenial confirmed that there the same case is working.