nbd: Miscellaneous typo fixes.

Signed-off-by: Hani Benhabiles <hani@linux.com>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>