Expand skip-halt logic to include aborted state

We already don't bother trying to halt a machine in the poweroff state.
This commit goes further and says that if a machine is aborted, we
aren't going to try to halt it. This is necessary to successfully do a
`vagrant snap rollback` against an aborted machine.
Merge pull request #6 from reidmv/avoid_halting_when_aborted

Expand skip-halt logic to include aborted state