kvm tools: Add helper to allocate and get TCP initial sequence number

kvm tools: Add helper to allocate and get TCP initial sequence number

Guest's initial sequence number can be found in the SYN package that
guest send to us to intialize a TCP session.

Remote server's initial sequence number is faked. RFC 793 specifies
that the ISN should be viewed as a 32-bit counter that increments
by one every 4 microseconds. For simplicity's sake, current
implementation in uip just returns a constant.

Signed-off-by: Asias He <asias.hejun@gmail.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>