Map libvirt error to InstanceNotFound in get_instance_disk_info

When getting instance disk info, the instance may be destroyed/deleted, and a
libvirtError will be raised in XMLDesc method, so catching and mapping it to
InstanceNotFound and the caller can handle it correctly.

Fixes bug #1064581

Change-Id: I07fed3e82e10dad4cb84ae5c8650ada351c24e78
Merge "Map libvirt error to InstanceNotFound in get_instance_disk_info"