target-i386: kvm_cpu_fill_host: use GET_SUPPORTED_CPUID

Change the kvm_cpu_fill_host() function to use
kvm_arch_get_supported_cpuid() instead of running the CPUID instruction
directly, when checking for supported CPUID features.

This should solve two problems at the same time:

 * "-cpu host" was not enabling features that don't need support on
   the host CPU (e.g. x2apic);
 * "check" and "enforce" options were not detecting problems when the
   host CPU did support a feature, but the KVM kernel code didn't
   support it.

Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
