blktap2: Fix build with gcc3. Cannot handle defining a function which

blktap2: Fix build with gcc3. Cannot handle defining a function which
is passed a struct-by-value which is not yet fully defined. Thus
defining a request struct which contains a pointer to a function which
is passed-by-value an instance of that request structure is
impossible. We work around it by defining the function poiinter as
void* and then casting in one place.

Signed-off-by: Keir Fraser <keir.fraser@citrix.com>