Commit 63d2dd2e ("vddk: Export block size information for this
plugin") tried to calculate the preferred block size from the logical
and physical sector size reported by VDDK.  Unfortunately VDDK < 7’s
VixDiskLib_GetInfo API returns a struct which does not contain
these fields at all.  We knew about this already because the
debug code does not print them, but the block size code uses
them regardless of the VDDK version.

The practical result of this error was that sometimes (depending on
existing contents of memory) you would see the error:

nbdkit: vddk[1]: error: plugin must set preferred block size to a power of 2

Fix this by only using the fields when VDDK >= 7, and in earlier
versions assuming VDDK’s normal sector size.

Reported-by: Xiaodai Wang
Fixes: commit 63d2dd2e
Fixes: https://bugzilla.redhat.com/show_bug.cgi?id=2066655