virtio-scsi: Don't abort when media is ejected

With an ejected block backend, blk_get_aio_context() would return
qemu_aio_context. In this case don't assert.

Signed-off-by: Fam Zheng <famz@redhat.com>
Message-Id: <1473848224-24809-3-git-send-email-famz@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>