qemu-nbd: Implement socket activation.

Socket activation (sometimes known as systemd socket activation)
allows an Internet superserver to pass a pre-opened listening socket
to the process, instead of having qemu-nbd open a socket itself.  This
is done via the LISTEN_FDS and LISTEN_PID environment variables, and a
standard file descriptor range.

This change partially implements socket activation for qemu-nbd.  If
the environment variables are set correctly, then socket activation
will happen automatically, otherwise everything works as before.  The
limitation is that LISTEN_FDS must be 1.

Signed-off-by: Richard W.M. Jones <rjones@redhat.com>
Message-Id: <20170204100317.32425-2-rjones@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
