Description of problem:
Depending on how the serial of a SCSI disk is configured and queried there are 3 possible length limits to which the serial is truncated:

1) when queried via VPD 0x80 -> 36 chars ( to fit UUID, raised from 20 to 36 in qemu 2.8)
2) when queried via VPD 0x83, configured via 'serial=' -> 20 chars (limit adopted from IDE?)
3) when queried via VPD 0x83, configured via 'device_id' -> 256 - 8 (message size limit)

Having multiple distinct arbitrary unjustified limit is wrong and caused problems such as when libvirt switched to using "device_id" which was required prior to change to use -blockdev.

See:

https://bugzilla.redhat.com/show_bug.cgi?id=1959997

To reproduce the issue, different behaviours happen when the 'serial=' is used alone or when 'device_id' is used.