virtio_blk: add discard and write zeroes support

In commit 88c85538, "virtio-blk: add discard and write zeroes features
to specification" (https://github.com/oasis-tcs/virtio-spec), the virtio
block specification has been extended to add VIRTIO_BLK_T_DISCARD and
VIRTIO_BLK_T_WRITE_ZEROES commands.  This patch enables support for
discard and write zeroes in the virtio-blk driver when the device
advertises the corresponding features, VIRTIO_BLK_F_DISCARD and
VIRTIO_BLK_F_WRITE_ZEROES.

Signed-off-by: Changpeng Liu <changpeng.liu@intel.com>
Signed-off-by: Daniel Verkamp <dverkamp@chromium.org>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>