tests/migration: Fail on unexpected migration states

tests/migration: Fail on unexpected migration states

We've got various places where we wait for a migration to enter
a given state; but if we enter an unexpected state we tend to fail
in odd ways; add a mechanism for explicitly testing for any state
which we shouldn't be in.

Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Message-Id: <20190923131022.15498-2-dgilbert@redhat.com>
Reviewed-by: Cleber Rosa <crosa@redhat.com>
Tested-by: Cleber Rosa <crosa@redhat.com>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>