ceilometer-agent-compute did not catch exception for disk error

When get instance disk info, ceilometer-agent-compute did not
catch exception if failed to get disk info caused by libvirt
error.

The fix was enable the exception handling for such case.

Fix bug 1209509

Change-Id: Ibcdcf3a7490a60b654f09e8aba487200e0c1d8fa
Merge "ceilometer-agent-compute did not catch exception for disk error"