Catch NoValidHost exception during live-migration

To avoid leaving the instance state as MIGRATING if scheduling of
a target host fails, add this exception to the list of exceptions
for which the instance task state is reset to None.

Resolves bug 1166771.

Change-Id: I23343091725051624c0eb0b82e9f03a7f9d5f4f4
Merge "Catch NoValidHost exception during live-migration"