virtio_blk: deprecate flush.

This is partially a port of Paolo Bonzini <pbonzini@redhat.com>'s
WCE spec changes, but puts them in the legacy section.  It reserves
the writeback field for compatibility.

Flush is now required.

See: https://lists.oasis-open.org/archives/virtio-comment/201308/msg00078.html

Signed-off-by: Rusty Russell <rusty@au1.ibm.com>
2.5.2.3:  document topology fields

From: Paolo Bonzini <pbonzini@redhat.com>

This completes the changes from yesterday.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

See:https://lists.oasis-open.org/archives/virtio-comment/201308/msg00030.html
Signed-off-by: Rusty Russell <rusty@au1.ibm.com>