KVM: Let host know whether the guest can handle async PF in non-userspace context.

If guest can detect that it runs in non-preemptable context it can
handle async PFs at any time, so let host know that it can send async
PF even if guest cpu is not in userspace.

Acked-by: Rik van Riel <riel@redhat.com>
Signed-off-by: Gleb Natapov <gleb@redhat.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>

arch/x86/include/asm/kvm_host.h

@@ -422,6 +422,7 @@ struct kvm_vcpu_arch {
 		struct gfn_to_hva_cache data;
 		u64 msr_val;
 		u32 id;
+		bool send_user_only;
 	} apf;
 };