Suppress or re-raise as NovaException for failed libvirt domain lookups

Libvirt exceptions should be handled locally by either suppressing or
re-raising them as NovaExceptions. This fix does:

- Fix unhandled exceptions in list_instance_uuids and get_memory_mb_used.
- Centralize libvirt exception handling in new method _lookup_by_id.
- Improve test coverage for list_instances and list_instance_uuids.

Resolves bug 1182184.

Change-Id: If946f9d05388c93c0630d454e5ca3b6625bb14f9