target-i386: Move kvm_check_features_against_host() check to realize time

kvm_check_features_against_host() should be called when features can't
be changed, and when features are converted to properties it would be
possible to change them until realize time, so correct way is to call
kvm_check_features_against_host() in x86_cpu_realize().

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>
