Improve prep_resize reschedule unit test
Improve prep_resize reschedule unit test

Unit test test_prep_resize_errors_migration was
failing on _get_scheduler_hints instead of
_prep_resize. This change fixes the unit test
to raise the error in the proper statement and
validate the intended code path.

Change-Id: Iadcc1944b43cc11d5e9d4c27e4949d811c2dcdfb

mahak/nova,openstack/nova,openstack/nova,mahak/nova,mahak/nova,openstack/nova