All current users (IDE, SCSI and virtio-blk) happen to share this 20
characters limit.  Still, it should be left to device models.  They
already enforce their limits.  They have to, as the DriveInfo limit
only affects legacy -drive serial=..., not the qdev properties.

usb-storage, which doesn't limit serial number length, also uses
DriveInfo for -usbdevice.  But that doesn't provide access to
DriveInfo serial.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>