util: Make some iova_tree parameters const

util: Make some iova_tree parameters const

As qemu guidelines:
Unless a pointer is used to modify the pointed-to storage, give it the
"const" attribute.

In the particular case of iova_tree_find it allows to enforce what is
requested by its comment, since the compiler would shout in case of
modifying or freeing the const-qualified returned pointer.

Signed-off-by: Eugenio Pérez <eperezma@redhat.com>
Acked-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20211013182713.888753-2-eperezma@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>