libvirt: add tests for _live_snapshot and _swap_volume methods

The _live_snapshot and _swap_volume methods do a non-trivial
sequence of libvirt API calls which should be validated in the
unit tests

Closes-bug: #1346327
Change-Id: I14bfa08f0dbb4eaa7c4f069f70c257ba7605989b
Merge "libvirt: add tests for _live_snapshot and _swap_volume methods"