block-backend: Allow more "can inactivate" cases

These two conditions corresponds to mirror job's source and target,
which need to be allowed as they are part of the non-shared storage
migration workflow: failing to inactivate either will result in a
failure during migration completion.

Signed-off-by: Fam Zheng <famz@redhat.com>
Message-Id: <20170823134242.12080-3-famz@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
[eblake: improve comment grammar]
Signed-off-by: Eric Blake <eblake@redhat.com>
2 files changed