target-i386: warns users when CPU threads>1 for non-Intel CPUs

Only Intel CPUs support hyperthreading. When users select threads>1 in
-smp option, QEMU fixes it by adjusting CPUID_0000_0001_EBX and
CPUID_8000_0008_ECX based on inputs (sockets, cores, threads);
so guest VM can boot correctly. However it is still better to gives
users a warning when such case happens.

Signed-off-by: Wei Huang <wei@redhat.com>
[As suggested by Eduardo, check for !IS_INTEL instead of AMD. - Paolo]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
