example.makecd

The capabilities are extensions to the CPU architecture's base instruction set.

New versions of software we care about such as the CentOS installer
requires "x86-64-v2" capabilities because glibc relies on them.

Also, Signal CLI (see #11)
apparently relies on them.

This is not the default setting in kvm in order to avoid issues with
live migration on some virtualization management systems, but
kvm-manager does not support live migration anyway, so this is
unlikely to become a problem.

see
https://www.qemu.org/docs/master/system/qemu-cpu-models.html#two-ways-to-configure-cpu-models-with-qemu-kvm
and
https://developers.redhat.com/blog/2021/01/05/building-red-hat-enterprise-linux-9-for-the-x86-64-v2-microarchitecture-level
for more details.

(dkg fleshed out these details, maxigas and jamie proposed the
concrete change)