exec: simplify phys_page_find() params

It really only plays with the dispatchers, so the parameter list does
not need that complexity. This helps for readability at least.

Signed-off-by: Peter Xu <peterx@redhat.com>
Message-Id: <1494838260-30439-2-git-send-email-peterx@redhat.com>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>