From KVM

In the section about running kvm as a non-root contains a setup that will not work. It says that one need to create a qemu-ifup script containing the line:

/usr/bin/sudo /usr/sbin/tunctl -u `whoami` -t $1

However, it will not work and result in an error-message when running qemu-system-x86_64. The command above needs to be run before qemu-system-x86_64.

I think this oneliner for generating a random MAC address is much more readable, and as an added bonus it will use the entire 16 bit space:

   printf "DE:AD:BE:EF:%02X:%02X\n" $((RANDOM&255)) $((RANDOM&255))

If you create the bridge manually ("brctl addbr br0") then you should also bring it up, with "ifconfig br0 up".