release: Use qemu when cross-building vm images

For a bit over 5 years, we have used qemu when cross-building cloudware
images; in particular, it's necessary when installing packages which
might include post-install scripts.

Use qemu in the vm-images target too; while "generic" vm images don't
install packages, they still run newaliases and /etc/rc.d/ldconfig,
both of which fail without appropriate emulation.

MFC after: 1 week