Fix wrong command for _rescan_multipath

The _run_multipath command is wrong in [1], it should be 'multipath -r'
not 'multipath - r'. And adds test for this.

[1]: https://github.com/openstack/nova/blob/master/nova/virt/libvirt/volume.py#L590

Change-Id: I3c5d12cc11988d65a8fc9e6143d26c2d1220d253
Closes-bug: 1362916
Merge "Fix wrong command for _rescan_multipath"