virtio-blk: add DISCARD and WRITE_ZEROES features

virtio-blk: add DISCARD and WRITE_ZEROES features

This patch adds the support of DISCARD and WRITE_ZEROES commands,
that have been introduced in the virtio-blk protocol to have
better performance when using SSD backend.

We support only one segment per request since multiple segments
are not widely used and there are no userspace APIs that allow
applications to submit multiple segments in a single call.

Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
Message-id: 20190221103314.58500-7-sgarzare@redhat.com
Message-Id: <20190221103314.58500-7-sgarzare@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>