run_archiso persistent disk option

IMHO it could be nice if run_archiso had persistent disk support? So one would be able to e.g. do something roughly like this:

cloud-init.sh
qemu-img create -f raw disk.raw 4G
run_archiso -v -i archlinux*.iso -c cloud-init.iso -p disk.raw
scp -P 60022 install.sh localhost:
ssh -p 60022 localhost install.sh
run_archiso -v -d -i disk.raw -c cloud-init.iso

I'll see if I can propose an MR with a concrete code proposal to review this idea with code instead of words.

@dvzrv @nl6720 @svenstaro @klausenbusk