xen-unstable/blkif: Add trim operation interface

xen-unstable/blkif: Add trim operation interface

Trim operation is a request for the underlying block device to mark
extents to be erased. Add the operation code and ring data structure
to the public header file.

Trim operations are passed with sector_number as the sector index to
begin trim operations at and nr_sectors as the number of sectors to
be trimmed. The specified sectors should be trimmed if the underlying
block device supports trim operations, or a BLKIF_RSP_EOPNOTSUPP
should be returned. More information about trim operations at;
http://t13.org/Documents/UploadedDocuments/docs2008/
    e07154r6-Data_Set_Management_Proposal_for_ATA-ACS2.doc

Signed-off-by: Owen Smith <owen.smith@citrix.com>