[vdsm] question about injectfile hook

[bigclouds]

hi, shaharh:
recently i work on injecting file into guestvm, i find a hook called  injectfile in vdsm.
i have a confusion about   why injectfile hook takes qcow2 format as a special one which
can not be handled?
could you tell me your reason?
 
as i know libguestfs certainly can be aware of qcow2 and all its feature like backing_file
 etc.
it is a little hard to inject files into a image which has backing files(maybe backing
chain)  especially for images of block type with thin-providing. like a vm in pool.
i am now writing code to inject files into a vm in pool.
please tell me your ideas. and my questoin.
 
thanks