KVM guest: do not batch pte updates from interrupt context

Commit b8bcfe99

 made paravirt pte updates synchronous in interrupt
context.

Unfortunately the KVM pv mmu code caches the lazy/nonlazy mode
internally, so a pte update from interrupt context during a lazy mmu
operation can be batched while it should be performed synchronously.

https://bugzilla.redhat.com/show_bug.cgi?id=518022

Drop the internal mode variable and use paravirt_get_lazy_mode(), which
returns the correct state.

Cc: stable@kernel.org
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
Signed-off-by: Avi Kivity <avi@redhat.com>