nbd: Shutdown socket before closing.

This forces finishing data sending to client before closing the socket like in
exports listing or replying with NBD_REP_ERR_UNSUP cases.

Signed-off-by: Hani Benhabiles <hani@linux.com>
Cc: qemu-stable@nongnu.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

Signed-off-by: Hani Benhabiles <kroosec@gmail.com>