xen-blkfront: remove frame list from blk_shadow

We already have the frame (pfn of the grant page) stored inside struct
grant, so there's no need to keep an aditional list of mapped frames
for a specific request. This reduces memory usage in blkfront.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Cc: xen-devel@lists.xen.org
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>