qemu-img: Gracefully shutdown when map can't finish

qemu-img: Gracefully shutdown when map can't finish

Trying 'qemu-img map -f raw nbd://localhost:10809' causes the
NBD server to output a scary message:

qemu-nbd: Disconnect client, due to: Failed to read request: Unexpected end-of-file before all bytes were read

This is because the NBD client, being remote, has no way to expose a
human-readable map (the --output=json data is fine, however). But
because we exit(1) right after the message, causing the client to
bypass all block cleanup, the server sees the abrupt exit and warns,
whereas it would be silent had the client had a chance to send
NBD_CMD_DISC. Other protocols may have similar cleanup issues, where
failure to blk_unref() could cause unintended effects.

Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <20190326184043.7544-1-eblake@redhat.com>
Reviewed-by: John Snow <jsnow@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>