cpuid: Fix multicore setup on Intel

cpuid: Fix multicore setup on Intel

The multicore CPUID code detects whether the guest is an Intel or an
AMD CPU, because the Linux kernel is picky about the CmpLegacy bit.
KVM by default passes through the host's vendor, which was not
catched by the code. So fork out the vendor determining bits into a
separate function to be used from both places and always get the real
vendor.
This fixes KVM's multicore setup on Intel CPUs.

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