I've read a few guides about this but they didn't work for me.
sudo apt-get install bridge-utils
#don't think the above is enough it won't work still even though you have by default an /etc/qemu-ifup that handles it if you have the right tools and setup
sudo qemu-system-x86_64 -net tap -net nic -enable-kvm -cpu host,vmx=on ~/VirtualBox\ VMs/vsphere-vcenter/vsphere-vcenter.vdi
W: /etc/qemu-ifup: no bridge for guest interface found
Add the following br0 adapter and make sure you replace eth0 with your network adapter name such as "enp3s0"
iface br0 inet dhcp
#restart your network/networking
sudo service networking restart
sudo qemu-system-x86_64 -m 11G -net tap -net nic -enable-kvm -cpu host,vmx=on ~/VirtualBox\ VMs/vsphere-vcenter/vsphere-vcenter.vdi
There is no more error or complaint about no bridge interface being found not that we've installed bridge utils and created a br0 bridge.