xen-blkfront: Provide for 'feature-flush-cache' the BLKIF_OP_WRITE_FLUSH_CACHE operation.

The operation BLKIF_OP_WRITE_FLUSH_CACHE has existed in the Xen
tree header file for years but it was never present in the Linux tree
because the frontend (nor the backend) supported this interface.

Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>