I cannot find any documentation about it... the only thing i've found is that i can pass custom options to qemu command line through the <qemu:commandline> tag, but, to my understanding, that would mean entirely remove the <disk> tag for the image and rewrite it as a list of args for the command line.

Isn't there an easier way?

An example of what i'm speaking about is: file=hd.qcow2,l2-cache-size=4194304,refcount-cache-size=262144