disable kvm cpu signature if x-vga is enabled

see
http://git.qemu.org/?p=qemu.git;a=commit;h=f522d2acc549dd11f495048330aa5f3f424a7dfa

last nvdia drivers don't install in kvm machine if they detect kvm signature.

This patch hide kvm signature in cpuflags (but don't disable kvm)

Signed-off-by: Alexandre Derumier <aderumier@odiso.com>
bump version to 3.1-28