Add test for HypervisorUnavailable on conductor

HypervisorUnavailable is added in excepted exception list in conductor
function migrate_server in commit d0a5e272, but the test cases are not added.
This patch adds test cases for it.

Change-Id: I83556e97cb151ff202b2377c00122a368b1c2892
Merge "Add test for HypervisorUnavailable on conductor"