tcp_output(): maximize LSO frames size

Maximizing the LSO packet size we reduce the CPU usage both in
the Guest, and in HV.

This improves the netperf TCP_STREAM test performance when it's executed vs
the netserver on the same host where we run an OSv Guest (8GB RAM, 1 vCPU)
by the following margins:

message size    compared to origin
64              30.18%
128             31.86%
256             59.60%
512             97.88%
1024            214.14%
2048            247.84%
4096            208.82%
8192            106.49%
16384           38.51%
32768           12.69%
65536           5.01%

Signed-off-by: Vlad Zolotarov <vladz@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>