Add ESX quality warning

In line with the other drivers we've been marking as untested,
this patch marks the ESX driver as "not tested" in the logs to
alert admins to the fact. The VCenter driver is being tested
with minesweeper, and thus does not emit a similar message on
startup.

Change-Id: I9217bab01761f68b716cdce3b21c2653abd9afa1
Merge "Add ESX quality warning"