Support Bridged Networking to enable guests accessible from LAN

Setting up bridged networking in KVM/QEMU is a huge pain. It requires understanding bridges, tap devices, interfaces and a lot of other Linux terminologies.

Here are some threads where people discuss their frustration in setting up bridges in KVM/QEMU.

https://unix.stackexchange.com/questions/118891/wireless-bridged-networking-in-kvm-why-is-it-so-complicated

https://superuser.com/questions/854172/creating-bridge-in-linux-share-network-like-virtualbox

https://superuser.com/questions/694929/wireless-bridge-on-kvm-virtual-machine

VirtualBox simplifies this by using a single dropdown where users can select bridge mode in Network Settings of the VM.

Gnome Boxes should provide an intuitive option to enable bridged networking which will enable VMs appear as devices in the local network. This will help users save time and messing with the system configuration files.