Set VNET_HW_INTERFACE_FLAG_SUPPORTS_TX_L4_CKSUM_OFFLOAD for the interface to skip checksum calculation if guest supports checksum offload.

This is needed because after interface_output checksum calculation, it turns off these flags

  b->flags &= ~VNET_BUFFER_F_OFFLOAD_TCP_CKSUM;

  b->flags &= ~VNET_BUFFER_F_OFFLOAD_UDP_CKSUM;

  b->flags &= ~VNET_BUFFER_F_OFFLOAD_IP_CKSUM;

vhost_user_output needs them to set the descriptor header. Since in this case, vhost_user will tell the driver that it needs to compute the checksum, why both to let interface_output do the checksum.