domain of virtual machine is blocked

sometimes the screen of domain freeze but i close the windows and reopen and i can operate.
sometimes i shutdown a domain, at end of shutdown the domain is blocked on running mode and i can only kill the process.
I attach the screenshot state of virt-manager and domain stopped.
if i try to force shutdown virt-manager generate this error:

Error shutting down domain: An error occurred, but the cause is unknown

Traceback (most recent call last):
  File "/usr/share/virt-manager/virtManager/asyncjob.py", line 96, in cb_wrapper
    callback(asyncjob, *args, **kwargs)
  File "/usr/share/virt-manager/virtManager/asyncjob.py", line 117, in tmpcb
    callback(*args, **kwargs)
  File "/usr/share/virt-manager/virtManager/domain.py", line 1085, in destroy
    self._backend.destroy()
  File "/usr/lib/python2.7/dist-packages/libvirt.py", line 733, in destroy
    if ret == -1: raise libvirtError ('virDomainDestroy() failed', dom=self)
libvirtError: An error occurred, but the cause is unknown

ProblemType: Bug
DistroRelease: Ubuntu 13.04
Package: virt-manager 0.9.4-2ubuntu3
ProcVersionSignature: Ubuntu 3.8.0-23.34-generic 3.8.11
Uname: Linux 3.8.0-23-generic x86_64
NonfreeKernelModules: nvidia
ApportVersion: 2.9.2-0ubuntu8.1
Architecture: amd64
Date: Fri Jun 7 21:19:46 2013
EcryptfsInUse: Yes
InstallationDate: Installed on 2012-11-18 (201 days ago)
InstallationMedia: Ubuntu 12.04.1 LTS "Precise Pangolin" - Release amd64 (20120817.1)
MarkForUpload: True
PackageArchitecture: all
SourcePackage: virt-manager
UpgradeStatus: Upgraded to raring on 2013-06-01 (5 days ago)