software/kvm: remove keyboard option

The keyboard option is actually not recommended in the QEMU
documentation
(https://manpages.debian.org/buster/qemu-system-x86/qemu-system-x86_64.1.en.html).
After some tests, it appears that noVNC is already sending raw keyCode
to QEMU since 2016 (see
https://github.com/novnc/noVNC/commit/f4f4e8993d6daeb43b957766cd6973993232ba13).

More information can be found in
https://github.com/novnc/noVNC/issues/21 and
https://www.berrange.com/posts/2010/07/04/more-than-you-or-i-ever-wanted-to-know-about-virtual-keyboard-handling/