VM builds: Use qemu instead of the obsolete qemu-kvm
VM builds: Use ext4 instead of ext2
VM builds: Panic on OOM
VM builds: Increase the default memory size to 512 MB
VM builds: Use 9p/virtfs instead of CIFS

9p (with caching enabled) is much faster than CIFS and doesn't require
Samba or virtual networking.  For instance, building GNU Hello with
CIFS takes ~323s on my laptop, but with 9p it takes 54s.

More measurements will be needed to see if "cache=fscache" is really
faster than "cache=loose" (the former seems to be a little bit
faster).
Remove runInGenericVM

It's not used anywhere and probably doesn't work anymore.