Avoid deadlock when stringifying NetworkInfo model

In the libvirt driver, we log information about the instance we're
about to generate XML for, which includes a NetworkInfo model. In
reality, this is a NetworkInfoAsyncWrapper which acquires a lock
on the model. Since we're in the middle of a log statement, we also
hold the logging lock. This happens right after we've fired off an
async request to update information about the instance, which first
acquired the network model lock and then acquired the logging lock
to make a seemingly innocuous log record.

The resulting deadlock is fixed by this patch by stringifying the
NetworkInfo object before making the log call in the libvirt
driver.

Change-Id: I81f35197ab7c74daa84fb51fbaa2df28c025da13
Closes-bug: 1276268
(cherry picked from commit 072e4ad20a6a7f4fff3b59eb7ef3f6fef9aa19d1)