block/qdev: Fix NULL access when using BB twice

BlockBackend has only a single pointer to its guest device, so it makes
sure that only a single guest device is attached to it. device-add
returns an error if you try to attach a second device to a BB. In order
to make the error message nicer, -device that manually connects to a
if=none block device get a different message than -drive that implicitly
creates a guest device. The if=... option is stored in DriveInfo.

However, since blockdev-add exists, not every BlockBackend has a
DriveInfo any more. Check that it exists before we dereference it.

QMP reproducer resulting in a segfault:

{"execute":"blockdev-add","arguments":{"options":{"id":"disk","driver":"file","filename":"/tmp/test.img"}}}
{"execute":"device_add","arguments":{"driver":"virtio-blk-pci","drive":"disk"}}
{"execute":"device_add","arguments":{"driver":"virtio-blk-pci","drive":"disk"}}

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
