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>
diff --git a/target-i386/cpu.h b/target-i386/cpu.h
index d7b32d4..3cd391a 100644
--- a/target-i386/cpu.h
+++ b/target-i386/cpu.h
@@ -655,6 +655,7 @@
     uint32_t cpuid_ext2_features;
     uint32_t cpuid_ext3_features;
     uint32_t cpuid_apic_id;
+    int cpuid_vendor_override;
 
     /* MTRRs */
     uint64_t mtrr_fixed[11];