sheepdog: add reopen support

With reopen supported, block-commit (and offline commit) is now supported for
image files whose base image uses the Sheepdog protocol driver.

Cc: qemu-devel@nongnu.org
Cc: Jeff Cody <jcody@redhat.com>
Cc: Kevin Wolf <kwolf@redhat.com>
Cc: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Liu Yuan <liuyuan@cmss.chinamobile.com>
Message-id: 1440730438-24676-1-git-send-email-namei.unix@gmail.com
Signed-off-by: Jeff Cody <jcody@redhat.com>