kvm: don't register smram_listener when smm is off

If the user set disable smm by '-machine smm=off', we
should not register smram_listener so that we can
avoid waster memory in kvm since the added sencond
address space.

Meanwhile we should assign value of the global kvm_state
before invoking the kvm_arch_init(), because
pc_machine_is_smm_enabled() may use it by kvm_has_mm().

Signed-off-by: Gonglei <arei.gonglei@huawei.com>
Message-Id: <1496316915-121196-1-git-send-email-arei.gonglei@huawei.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>