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: [email protected]
Cc: Markus Armbruster <[email protected]>
Cc: Kevin Wolf <[email protected]>
Cc: Stefan Hajnoczi <[email protected]>
Reported-by: Markus Armbruster <[email protected]>
Signed-off-by: Liu Yuan <[email protected]>
Message-id: 1424231875[email protected]
Signed-off-by: Stefan Hajnoczi <[email protected]>
Signed-off-by: Kevin Wolf <[email protected]>