Testing the new patch using an OmniOS guest:
-s 4:0,virtio-blk,/dev/zvol/rdsk/data/vm/bhyvetest/root,serial=918273645
and also in the configuration file format of:
pci.0.4.0.device=virtio-blk
pci.0.4.0.path=/dev/zvol/rdsk/data/vm/bhyvetest/root
pci.0.4.0.serial=918273645
the guest sees the manually configured serial number:
root@omnios:~# iostat -En
c1t0d0 Soft Errors: 0 Hard Errors: 0 Transport Errors: 0
Vendor: Virtio Product: Block Device Revision: 0000 Serial No: 918273645
Size: 8.59GB <8589934592 bytes>
Media Error: 0 Device Not Ready: 0 No Device: 0 Recoverable: 0
Illegal Request: 0 Predictive Failure Analysis: 0
The same thing works as expected using ser in place of serial (for consistency with AHCI and NVME, which both use ser).
Updated by 
Updated by