Do not pass instances without host to compute API

Even if the server external events extension filters out events
whose related instance does not have a host, the corresponding
instance is still sent to the compute API module.
As this might result in KeyError, instance without host should
be filtered out before calling the compute API module.

Change-Id: If5229ec3059076dbc9f4abb6625504e8864c265e
Closes-Bug: #1348584
(cherry picked from commit 5e4a5f0d8c62ca6e94ae6db16e9fbe0428805158)
Merge "Do not pass instances without host to compute API" into stable/icehouse