qemu: Add capability for CFPC pSeries feature

qemu: Add capability for CFPC pSeries feature

CFPC (Cache Flush on Privilege Change) is one of the capabilities
added to QEMU to mitigate Spectre vulnerabilities in Power chips.
It was implemented in QEMU 2.12 by commit 6898aed77f46.

This capability is still used today due to differences in how
the host setup (hardware and firmware/kernel) can handle this
mitigation. Its default value also varies with the pseries machine
version of the time. There's also certain OSes, like AIX, that
might not support the default value of the pseries machine the
guest uses.

Exposing this in the Libvirt XML as a feature will allow users to tune
CFPC values in a cleaner way, instead of hacking parameters in
<qemu:commandline> elements.

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