Raise exceptions when Spice/VNC are unavailable

Bug 1192724

Both Spice and/or VNC may be enabled from the perspective of compute
configuration but their actual availability can vary based on
whether either was enabled/disabled at the time the instance was
launched. This patch raises a proper exception in this case, such
that clients (including Horizon) can either message the user or fall
back.

Change-Id: If06e726e606f4a27c8ec4617400b728ce7112e3b
Merge "Raise exceptions when Spice/VNC are unavailable"