target-i386: kvm_cpu_fill_host(): No need to check xlevel2

There's no need to check CPU xlevel2 before calling
kvm_arch_get_supported_cpuid(s, 0xC0000001, 0, R_EDX), because:

 * The kernel won't return any entry for 0xC0000000 if host CPU vendor
   is not Centaur (See kvm_dev_ioctl_get_supported_cpuid() on the kernel
   code)
 * Similarly, the kernel won't return any entry for 0xC0000001 if
   CPUID[0xC0000000].EAX is < 0xC0000001
 * kvm_arch_get_supported_cpuid() will return 0 if no entry is returned
   by the kernel for the requested leaf

For similar reasons, we can simply set x86_cpu_def->xlevel2 directly
instead of making it conditional, because it will be set to 0 CPU vendor
is not Centaur.

This will simplify the kvm_cpu_fill_host() code a little.

Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
[Remove unparseable comment. - Paolo]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
