libvirt: Fix confusing use of mox.StubOutWithMock

The code was using a side effect of mox.StubOutWithMock to reset
libvirt_driver.LibvirtDriver._conn after a test before resetting it manually.
This code is shorter, and it's obvious what it does.

Change-Id: I78393f76e132397dd198ac4b6ff998c785ff5690
Merge "libvirt: Fix confusing use of mox.StubOutWithMock"