If a guest accesses swapped out memory do not swap it in from vcpu thread
context. Schedule work to do swapping and put vcpu into halted state
instead.

Interrupts will still be delivered to the guest and if interrupt will
cause reschedule guest will continue to run another task.

[avi: remove call to get_user_pages_noio(), nacked by Linus; this
makes everything synchrnous again]

Acked-by: Rik van Riel
Signed-off-by: Gleb Natapov
Signed-off-by: Marcelo Tosatti