target-i386: Move KVM default-vendor hack to instance_init

As we will not have a cpu_x86_find_by_name() function anymore,
move the KVM default-vendor hack to instance_init.

Unfortunately we can't move that code to class_init because it depends
on KVM being initialized.

Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
