Description of problem:
I created a gluster backend on virt-manager and I cannot create new virtual disks.
But I can see the content of the gluster volume and load the ISO file, only the creation of the disk fails.


Version-Release number of selected component (if applicable):
CentOS Linux release 7.4.1708 (Core)
virt-manager-1.4.1-7.el7.noarch
virt-manager-common-1.4.1-7.el7.noarch
virt-viewer-5.0-7.el7.x86_64
libvirt-3.2.0-14.el7_4.9.x86_64
qemu-system-x86-2.0.0-1.el7.6.x86_64
qemu-img-1.5.3-141.el7_4.6.x86_64
qemu-kvm-1.5.3-141.el7_4.6.x86_64


How reproducible:

Steps to Reproduce:
1. yum -y install kvm libvirt qemu-kvm libvirt-python virt-manager virt-viewer virt-install

2. Make sure that the gluster volume has the right permissions
  gluster volume set gkvms server.allow-insecure on
  ## Adjust ownership to qemu user
  gluster volume set gkvms storage.owner-uid 107 
  gluster volume set gkvms storage.owner-gid 107

3. Create a gluster pool (I removed the default one and called this one "default")

4. Try to create a new VM and click on "Forward" on the 4th step (Create a disk image for the virtual machine)



Actual results:

When clicking on "Forward" on step 4 of 5 of "Create a new virtual machine", it pops up this error message:

Uncaught error validating install parameters: 'NoneType' object has no attribute 'endswith'

Traceback (most recent call last):
  File "/usr/share/virt-manager/virtManager/create.py", line 1697, in _validate
    return self._validate_storage_page()
  File "/usr/share/virt-manager/virtManager/create.py", line 1963, in _validate_storage_page
    self._guest.name, do_log=True)
  File "/usr/share/virt-manager/virtManager/create.py", line 1955, in _get_storage_path
    path = self._addstorage.get_default_path(vmname)
  File "/usr/share/virt-manager/virtManager/addstorage.py", line 238, in get_default_path
    path = os.path.join(target, path)
  File "/usr/lib64/python2.7/posixpath.py", line 77, in join
    elif path == '' or path.endswith('/'):
AttributeError: 'NoneType' object has no attribute 'endswith'



Expected results:

The virtual disk should be created on the gluster volume.

Additional info:

> virsh pool-info default
Name:           default
UUID:           80061946-963c-4afc-823a-fea6d4c15241
State:          running
Persistent:     yes
Autostart:      yes
Capacity:       3,21 TiB
Allocation:     8,38 GiB
Available:      3,20 TiB


> virsh pool-dumpxml default
<pool type='gluster'>
  <name>default</name>
  <uuid>80061946-963c-4afc-823a-fea6d4c15241</uuid>
  <capacity unit='bytes'>3528313929728</capacity>
  <allocation unit='bytes'>8996388864</allocation>
  <available unit='bytes'>3519317540864</available>
  <source>
    <host name='my-server.example.com'/>
    <dir path='/VMs'/>
    <name>kvms</name>
  </source>
</pool>


> qemu-img create -f qcow2 gluster://my-server.example.com/kvms/VMs/test.qcow2 1G
Formatting 'gluster://my-server.example.com/kvms/VMs/test.qcow2', fmt=qcow2 size=1073741824 encryption=off cluster_size=65536 lazy_refcounts=off debug=0 
[2018-04-12 13:27:55.579560] E [MSGID: 108006] [afr-common.c:5006:__afr_handle_child_down_event] 0-kvms-replicate-0: All subvolumes are down. Going offline until atleast one of them comes back up.
[2018-04-12 13:27:56.682912] E [MSGID: 108006] [afr-common.c:5006:__afr_handle_child_down_event] 0-kvms-replicate-0: All subvolumes are down. Going offline until atleast one of them comes back up.
[2018-04-12 13:27:57.593471] E [MSGID: 108006] [afr-common.c:5006:__afr_handle_child_down_event] 0-kvms-replicate-0: All subvolumes are down. Going offline until atleast one of them comes back up.

> virsh vol-list default
# Does not show the image, but by mounting the volume with "mount" I can see that it was actually created.

# Additional info:
- The gluster bricks on each gluster node are created on a zfs volumes.
- I can import VM images that are already on the pool, which I copied with "cp" and they work fine.