Add a BlockBackend member to TrimAIOCB, so ide_issue_trim_cb() can use
blk_aio_discard() instead of bdrv_aio_discard().

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>