pvscsi: Advertise maxio of 256k.

While the PV SCSI SG list can handle 512k of SG entries, it can only do
so for I/O that's aligned to 4k or better. newfs_msdos does unaligned
I/O, so triggers too long for host errors in cam when a 512k I/O is
attempted. Prefer power of 2 256k to the absolute maximum 508k, though
that can be revisited should the latter show to give significant
performance improvement.

MFC After: 3 days
Tested by: darius on discord (508k version of patch)
Sponsored by: Netflix