qemu: remove nested branching to enhance readability

qemu: remove nested branching to enhance readability

This is a follow-up to patch series posted in

https://www.redhat.com/archives/libvir-list/2019-November/msg01180.html

It implements a suggestion made by Cole in

https://www.redhat.com/archives/libvir-list/2019-November/msg01207.html

and discussed in follow-up messages as there were no objections to the
change.

The aim is to make the code more readable by replacing nested branching
with a flat structure.

Signed-off-by: Pavel Mores <pmores@redhat.com>
Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Daniel Henrique Barboza <danielhb413@gmail.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>