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 foun........