Description of problem:
Create a qcow2 format image in default libvirt pool and let it point at an iscsi storage(/dev/sdb) or USB stoage. Then logout the iscsi or USB storage. When create a new guest via virt-manager, the Browse dialog of "Select managed or other existing storage" can not open.
BTW, check the Connection Detail and select the Storage tab, the default pool only show up an error message:
"Error selecting pool: cannot stat file '/dev/sdb':No such file or directory"
maybe this's an expect result since there's an broken image locate at this pool.


Version-Release number of selected component (if applicable):


How reproducible:
libvirt-0.9.8-1.el6.x86_64
virt-manager-0.9.0-7.el6.x86_64


Steps to Reproduce:
1. Create a qcow2 format image and point at an iscsi storage
#qemu-img create -f qcow2 -o backing_file=/dev/sdb /var/lib/libvirt/images/test.qcow2

2. Logout the storage
iscsiadm -m node -T iqn.2011-12.com.redhat:ydu.target0 --logout

3. Create a new guest via virt-manager, and browse existing storage for the guest.
  
Actual results:
Step3, Browse dialog can't open, and --debug output the following message:

Traceback (most recent call last):
  File "/usr/share/virt-manager/virtManager/create.py", line 1240, in browse_storage
    self._browse_file(set_path, is_media=False)
  File "/usr/share/virt-manager/virtManager/create.py", line 2122, in _browse_file
    self.storage_browser.show(self.topwin, self.conn)
  File "/usr/share/virt-manager/virtManager/storagebrowse.py", line 74, in show
    self.reset_state(conn)
  File "/usr/share/virt-manager/virtManager/storagebrowse.py", line 183, in reset_state
    self.pool_selected()
  File "/usr/share/virt-manager/virtManager/storagebrowse.py", line 257, in pool_selected
    self.populate_storage_volumes()
  File "/usr/share/virt-manager/virtManager/storagebrowse.py", line 322, in populate_storage_volumes
    path = vol.get_target_path()
  File "/usr/share/virt-manager/virtManager/storagevol.py", line 49, in get_target_path
    return util.xpath(self.get_xml(), "/volume/target/path")
  File "/usr/share/virt-manager/virtManager/libvirtobject.py", line 93, in get_xml
    self.refresh_xml()
  File "/usr/share/virt-manager/virtManager/libvirtobject.py", line 105, in refresh_xml
    self._xml = self._XMLDesc(self._active_xml_flags)
  File "/usr/share/virt-manager/virtManager/storagevol.py", line 35, in _XMLDesc
    return self.vol.XMLDesc(flags)
  File "/usr/lib64/python2.6/site-packages/libvirt.py", line 2002, in XMLDesc
    if ret is None: raise libvirtError ('virStorageVolGetXMLDesc() failed', vol=self)
libvirt.libvirtError: cannot stat file '/dev/sdb': No such file or directory

Expected results:
Browse dialog of "Select managed or other existing storage" can open.

Additional info:
There's a workaround. 
If there's a boot-scratch libvirt pool, the dialog can open, and all volumes expect the broken image can be listed out.
# virsh pool-list
Name                 State      Autostart 
-----------------------------------------
boot-scratch         active     yes       
default              active     yes       

# virsh pool-dumpxml boot-scratch
<pool type='dir'>
  <name>boot-scratch</name>
  <uuid>8f8f4527-7600-7a67-e2f3-2a3a016cf49d</uuid>
  <capacity>42275766272</capacity>
  <allocation>2492018688</allocation>
  <available>39783747584</available>
  <source>
  </source>
  <target>
    <path>/var/lib/libvirt/boot</path>
    <permissions>
      <mode>0700</mode>
      <owner>-1</owner>
      <group>-1</group>
    </permissions>
  </target>
</pool>