I have finally found the solution here:

https://www.mail-archive.com/search?l=ubuntu-bugs@lists.ubuntu.com&q=subject:%22%5C%5BBug+405388%5C%5D+Re%5C%3A+virt%5C-manager+cannot+create+image+file%22&o=newest&f=1

By default is qemu:///system activated but I had to change to qemu:///session because I was getting errors. I created a VM with qemu:///session and it was running without any issues. But it didn't work for ZFS Volums. I suppose the other change that it was necessary is set the user to root in the /etc/libvirt/qemu.conf. I have created a new volume in VMM and installed Fedora. Everything seems to be working fine inclusive ZFS snapshots.

Regards,

Nick

On Wed, Oct 4, 2017 at 4:58 PM, Peter Krempa <pkrempa@redhat.com> wrote:

On Tue, Oct 03, 2017 at 08:44:10 +0200, Michal Privoznik wrote:
> On 10/02/2017 11:42 PM, Nick Gilmour wrote:

[...]

>
> 2) storage backends are static. While we have libvirt drivers in
> separate modules (.so files) and load them at start up, it's not the
> same story with storage driver backends. These are hardcoded into the
> storage driver. Might be worth it to have them dynamically loadable too.

This was changed some time ago. Backends are loadable now:

src/.libs/libvirt_storage_backend_disk.so
src/.libs/libvirt_storage_backend_fs.so
src/.libs/libvirt_storage_backend_gluster.so
src/.libs/libvirt_storage_backend_iscsi.so
src/.libs/libvirt_storage_backend_logical.so
src/.libs/libvirt_storage_backend_mpath.so
src/.libs/libvirt_storage_backend_rbd.so
src/.libs/libvirt_storage_backend_scsi.so
src/.libs/libvirt_storage_backend_sheepdog.so
src/.libs/libvirt_storage_backend_vstorage.so
src/.libs/libvirt_storage_backend_zfs.so

Similarly to driver backends they are loaded at start of libvirtd.