Make resize raise exception when no valid host found

When resize operation starts, it will ask conductor to find a
valid host and try to resize to that host. When there is no valid
host can be found ,current code return directly without error
notification, which will make upper layer code (e.g novaclient)
thinks the execution is succesfully performed and report 'Finished'.
But actually it isn't due to no valid host was found.

Change-Id: I3d66dbfc0a7346762b920e802aa3be92c43d93b7
Closes-Bug: #1297052
Merge "Make resize raise exception when no valid host found"