KVM: Clear async page fault hash after switching to real mode

The hash array of async gfns may still contain some left gfns after
kvm_clear_async_pf_completion_queue() called, need to clear them.

Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Acked-by: Gleb Natapov <gleb@redhat.com>
Signed-off-by: Avi Kivity <avi@redhat.com>