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

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 <[email protected]>
Signed-off-by: Gleb Natapov <[email protected]>
Signed-off-by: Marcelo Tosatti <[email protected]>