introduce kvm64 CPU

In addition to the TCG based qemu64 type let's introduce a kvm64 CPU type,
which is the least common denominator of all KVM-capable x86-CPUs
(based on Intel Pentium 4 Prescott). It can be used as a base type
for migration.

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