Not getting gigabit throughput in VM

We have a Dell R720 with multiple gigabit adapters. Initially, our CentOS 7 VM came up with rtl8139 driver, and it ran at just 100Mbps, so we have tried virtio and e1000 drivers, however, and we are still stuck at 100Mbps nominal throughput. The underlying hardware is more than capable of filling 1 Gbps. Is there some secret I am missing to getting above 100Mbps on the guest? I am testing this by doing scp’s from the guest to a specific server. The host R720 can do the scp at gigabit, but the same command on the guest never gets out of the 100Mbps range.