libvirt: convert mox to mock in test_utils

Commit 73066aff1b4c80f09a1340ab5d70b3b4f2983c0b created a new
file called test_utils. This was a good place to change
all mox usage to mock.

Change-Id: Iccf7c71d357df0c967f8745c56dcd41219f4ca87
Merge "libvirt: convert mox to mock in test_utils"