Description of problem:
when using a conversion host VM, based on downstream UCI image (RHEL8.1 based):
v2v-conversion-host-appliance-5.11.4.1-1_scratch.x86_64.qcow2,
VM v2v migration from vSphare 6.5 (storage: ISCSI) to RHV 4.3.7 (storage: FCP) is much slower, specifically, when using VDDK 6.7.1.

For example, VM with 100GB, 66% usage, it took 40 minutes to migrate, when using VDDK 6.7.1,
while it took 14 minutes, when using VDDK 6.5.2.

For VDDK 6.5.2,
v2v log contain this line:
"nbdkit: vddk[1]: debug: can_extents: VixDiskLib_QueryAllocatedBlocks == NULL, probably this is VDDK < 6.7"

For VDDK 6.7.1,
The above line is not mentioned in the v2v.log,
instead there are a lot of messages like this one:
nbdkit: python[1]: debug: can_extents
nbdkit: python[1]: debug: newstyle negotiation: flags: export 0xcd
nbdkit: python[1]: debug: newstyle negotiation: NBD_OPT_GO: ignoring NBD_INFO_* request 3 (unknown)
nbdkit: python[1]: debug: handshake complete, processing requests serially
nbdkit: vddk[3]: debug: extents count=196608 offset=851968 req_one=1
nbdkit: vddk[3]: debug: VDDK call: VixDiskLib_QueryAllocatedBlocks (handle, 1664 sectors, 384 sectors, 128 sectors)
nbdkit: vddk[3]: debug: VixDiskLib: VixDiskLib_QueryAllocatedBlocks: Query allocated blocks.
nbdkit: vddk[3]: debug: VDDK call: VixDiskLib_FreeBlockList (block_list)

When using a RHV physical RHEL 7.7 host, as a conversion host,
VM v2v migration from vSphare 6.5 (storage: ISCSI) to RHV 4.3.7 (storage: FCP) took 16 minutes, when using VDDK 6.7.1.
That is, the slow VM migration was not seen on this case.

Additional info:
According to this VMware doc, VDDK 6.7.1 has backwards support for vSphere 6.5 and 6.0:
https://vdc-download.vmware.com/vmwb-repository/dcr-public/d541510b-164a-4a32-9a1d-883004b5dff9/bc74fac6-9b4a-4350-91ed-9f6e8d7b49db/vsphere-vddk-671-release-notes.html

Also slowness is mentioned in this doc, under title:
"NBD transport in VDDK 6.7 is slow when running against vSphere 6.5.",
however it is stated as resolved for VDDK 6.7.1.