remove CPUID host hacks

KVM provides an in-kernel feature to disable CPUID bits that are not
present in the current host. So there is no need here to duplicate this
work. Additionally allows 3DNow! on capable processors, since the
restriction seems to apply to QEMU/TCG only.

Signed-off-by: Andre Przywara <andre.przywara@amd.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
