qdev: Decouple qdev_prop_drive from DriveInfo

Make the property point to BlockDriverState, cutting out the DriveInfo
middleman.  This prepares the ground for block devices that don't have
a DriveInfo.

Currently all user-defined ones have a DriveInfo, because the only way
to define one is -drive & friends (they go through drive_init()).
DriveInfo is closely tied to -drive, and like -drive, it mixes
information about host and guest part of the block device.  I'm
working towards a new way to define block devices, with clean
host/guest separation, and I need to get DriveInfo out of the way for
that.

Fortunately, the device models are perfectly happy with
BlockDriverState, except for two places: ide_drive_initfn() and
scsi_disk_initfn() need to check the DriveInfo for a serial number set
with legacy -drive serial=...  Use drive_get_by_blockdev() there.

Device model code should now use DriveInfo only when explicitly
dealing with drives defined the old way, i.e. without -device.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

hw/s390-virtio.c

@@ -262,7 +262,7 @@ static void s390_init(ram_addr_t ram_size,
         }
 
         dev = qdev_create((BusState *)s390_bus, "virtio-blk-s390");
-        qdev_prop_set_drive(dev, "drive", dinfo);
+        qdev_prop_set_drive(dev, "drive", dinfo->bdrv);
         qdev_init_nofail(dev);
     }
 }