virtio-blk: add discard and write zeroes features to specification

Existing virtio-blk protocol doesn't have DISCARD/WRITE ZEROES support,
this will impact the performance when using SSD backend over file systems.

Here is the proposal to extend existing virtio-blk protocol to support
DISCARD/WRITE ZEROES commands.

Basic idea here is using 16 Bytes payload to support 1 descriptor, users
can put several segments together with 1 DISCARD/WRITE ZEROES command.

struct virtio_blk_discard_write_zeroes {
       le64 sector;
       le32 num_sectors;
       struct {
               le32 unmap:1;
               le32 reserved:31;
       } flags;
};

For the purpose to support such feature, we need to introduce 2 new feature
flags: VIRTIO_BLK_F_DISCARD/VIRTIO_BLK_F_WRITE_ZEROES, and 2 new command
types: VIRTIO_BLK_T_DISCARD/VIRTIO_BLK_T_WRITE_ZEROES. Also we introduce
several new parameters in the configuration space of virtio-blk:
max_discard_sectors/max_discard_seg/max_write_zeroes_sectors.
These parameters will tell the OS what's the granularity when
issuing such commands.

If both DISCARD and WRITE ZEROES are supported, unmap flag bit maybe used
for WRITE ZEROES command with DISCARD bit enabled.

Signed-off-by: Changpeng Liu <changpeng.liu@intel.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Approved-by: https://www.oasis-open.org/apps/org/workgroup/virtio/ballot.php?id=3181
Fixes: https://github.com/oasis-tcs/virtio-spec/issues/5

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