target-i386: Move xcc->kvm_required check to realize time

It will allow to drop custom cpu_x86_init() and use
cpu_generic_init() instead, reducing cpu_x86_create()
to a simple 3-liner.

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
