Mask block_device_info auth_password in virt driver debug logs

The block_device_info object can have an auth_password key which is
getting logged at debug level in several virt drivers so we need to
sanitize the message getting logged.

Adds tests to ensure the logged messages are properly sanitized.

Note that bug 1321785 was opened to track the long-term design issues
with storing the password in the block_device_info dict since this can
crop up elsewhere if it's logged.  The immediate fix here is to mask
what's already exposed.

Closes-Bug: #1319943

Change-Id: I0eae07ce3f0f39861eb97ec3dec44895386c7d04
Merge "Mask block_device_info auth_password in virt driver debug logs"