virtio-scsi: don't crash without a valid device

Make sure that we actually have a device when checking the aio
context. Otherwise guests could trigger QEMU crashes.

Signed-off-by: "Eugene (jno) Dvurechenski" <jno@linux.vnet.ibm.com>
Reviewed-by: David Hildenbrand <dahi@linux.vnet.ibm.com>
Message-Id: <1448549135-6582-2-git-send-email-jno@linux.vnet.ibm.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
