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
Merge "Do not pass instances without host to compute API"