KVM: Send async PF when guest is not in userspace too.

If guest indicates that it can handle async pf in kernel mode too send
it, but only if interrupts are enabled.

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