The 'TRIM' command from VM that is to release underlying data storage for
better thin-provision is already supported by the Sheepdog.
This patch adds the TRIM support at QEMU part.
For older Sheepdog that doesn't support it, we return 0(success) to upper layer.
Cc: MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp> Cc: Kevin Wolf <kwolf@redhat.com> Cc: Stefan Hajnoczi <stefanha@redhat.com> Cc: Paolo Bonzini <pbonzini@redhat.com> Signed-off-by: Liu Yuan <tailai.ly@taobao.com> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>