The client process right now uses SIGTERM to interrupt the server side.
This does not affect the exit status of "qemu-nbd -v -c" because the
server is a child process.  This will change when both sides will be
in the same process, and anyway cleaning up things nicely upon SIGTERM
is good practice.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>