qemu-nbd: fix socket creation race

Now that the client and server are in the same process, there is
no need to race on the creation of the socket.  We can open the
listening socket before starting the client thread.

This avoids that "qemu-nbd -v -c" prints this once before connecting
successfully to the socket:

    connect(unix:/var/lock/qemu-nbd-nbd0): No such file or directory

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