Fix exception handling miss in remote_consoles.

The ConsoleTypeUnavailable exception raised in
nova/virt/libvirt/driver.py is not handled, and will make
nova api and compute fail.
This patch fixes this bug.

Closes-bug: #1270076

Change-Id: I6ff7a40bf26dbef4bd5512a9c3c4a8664a664244
Merge "Fix exception handling miss in remote_consoles."