omit 3DNOW! CPUID bits from qemu64 CPU model

Since we recently do not disable 3DNOW! support anymore, we should
avoid setting the bits in the default qemu64 CPU model to ease
migration. TCG does not support it anyway and even AMD deprecates
it's usage nowadays.
If you want to use it in KVM, use the phenom, athlon or host CPU
model.
Signed-off-by: NAndre Przywara <andre.przywara@amd.com>
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>

target-i386/helper.c

@@ -133,8 +133,7 @@ static x86_def_t x86_defs[] = {
             CPUID_PSE36,
         .ext_features = CPUID_EXT_SSE3,
         .ext2_features = (PPRO_FEATURES & 0x0183F3FF) | 
-            CPUID_EXT2_LM | CPUID_EXT2_SYSCALL | CPUID_EXT2_NX |
-            CPUID_EXT2_3DNOW | CPUID_EXT2_3DNOWEXT,
+            CPUID_EXT2_LM | CPUID_EXT2_SYSCALL | CPUID_EXT2_NX,
         .ext3_features = CPUID_EXT3_SVM,
         .xlevel = 0x8000000A,
         .model_id = "QEMU Virtual CPU version " QEMU_VERSION,