Add note on CVE-2017-5715 for qemu

Add a note on Qemu update. To protect from CVE-2017-5715 one would need
to have an 1/ intel / amd cpu microcode update, 2/ a qemu update to pass
new MSR and CPU flags from the microcode update 3/ host kernel update.

Refer to the relevant part for Qemu:

https://lists.nongnu.org/archive/html/qemu-devel/2018-01/msg00811.html