xen/blk{front,back}: fix usage of sector sizes different than 512b

The units of the size reported in the 'sectors' xenbus node is always 512b,
regardless of the value of the 'sector-size' node. The sector offsets in
the ring requests are also always based on 512b sectors, regardless of the
'sector-size' reported in xenbus.

Fix both blkfront and blkback to assume 512b sectors in the required fields.

The blkif.h public header has been recently updated in upstream Xen repository
to fix the regressions in the specification introduced by later modifications,
and clarify the base units of xenstore and shared ring fields.

PR: 280884
Reported by: Christian Kujau
MFC after: 1 week
Sponsored by: Cloud Software Group
Reviewed by: markj
Differential revision: https://reviews.freebsd.org/D46756

(cherry picked from commit e7fe85643735ffdcf18ebef81343eaac9b8d2584)