Clarify virt driver test comments & log statement

The current implementation of nova.tests.virt.test_virt_drivers
indicates NotImplementedError should be caught and logged to
dynamically build the hypervisor support matrix. This is not a
realistic / optimal approach for building the support matrix and
thus this patch updates the code comments accordingly for
clarity. This patch also modifies the logging statement outputted
in such scenarios to better clarify which test case method is
executing.

 Partial-Bug: 1209421

Change-Id: I5d80cdc06207071548fdb5f6852ca96fa8d79776
Merge "Clarify virt driver test comments & log statement"