Revert XenApi virt driver should throw exception

This reverts commit 2cb31123302a1ec8c8dec0cb2ac288a2e3b19c68.

Fixes LP Bug #1194264.

Change-Id: Ib8f8a8cfdcc1b37bb87fee3983081154a4fa2b2b
Merge "Revert XenApi virt driver should throw exception"