libvirt: ignore false exception due to slow NFS on resize-revert

When resize revert one VM instance, if the NFS is slow, the NFS
mounted on source host might not respond quickly. This will cause
that even if the VM instance configuration files was deleted from
resize target host, the NFS on source host may still think the
files were not deleted. Then in driver.py::finish_revert_migration,
the driver try to delete the file again, this will lead nova compute
throw exception.

The fix was ignore exception for such case: if failed to delete the
instance with error no as 2 (No such file or directory)

Fix bug 1206312

Change-Id: I89990c1750a7fd7f421cab03780e7704ee661de1
Merge "libvirt: ignore false exception due to slow NFS on resize-revert"