blkif.h: Define and document the request number/size/segments extension

blkif.h: Define and document the request number/size/segments extension

Note: As of __XEN_INTERFACE_VERSION__ 0x00040201 the definition of
      BLKIF_MAX_SEGMENTS_PER_REQUEST has changed.  Drivers must be
      updated to, at minimum, use BLKIF_MAX_SEGMENTS_PER_HEADER_BLOCK,
      before being recompiled with a __XEN_INTERFACE_VERSION greater
      than or equal to this value.

This extension first appeared in the FreeBSD Operating System.

Signed-off-by: Justin T. Gibbs <justing@spectralogic.com>
Committed-by: Keir Fraser <keir@xen.org>