sheepdog: fix confused return values

sheepdog: fix confused return values

These functions mix up -1 and -errno in return values and would might cause
trouble error handling in the call chain.

This patch let them return -errno and add some comments.

Cc: qemu-devel@nongnu.org
Cc: Markus Armbruster <armbru@redhat.com>
Cc: Kevin Wolf <kwolf@redhat.com>
Cc: Stefan Hajnoczi <stefanha@redhat.com>
Reported-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Liu Yuan <liuyuan@cmss.chinamobile.com>
Message-id: 1424231875-7131-1-git-send-email-namei.unix@gmail.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>