KVM: x86: allow KVM_STATE_NESTED_MTF_PENDING in kvm_state flags

The migration functionality was left incomplete in commit 5ef8acbd
("KVM: nVMX: Emulate MTF when performing instruction emulation", 2020-02-23),
fix it.

Fixes: 5ef8acbd

 ("KVM: nVMX: Emulate MTF when performing instruction emulation")
Cc: stable@vger.kernel.org
Reviewed-by: Oliver Upton <oupton@google.com>
Reviewed-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>