allow CPUID vendor override

KVM-enabled QEMU will always report the vendor ID of the physical CPU it is
running on. Allow to override this if explicitly requested on the
command line. It will not suffice to name a CPU type (like -cpu phenom),
but you have to explicitly set the vendor: -cpu phenom,vendor=AuthenticAMD

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