exec.c: Pass correct pointer type to qemu_ram_ptr_length

Commit e3127ae0 introduced a problem where we're passing a
hwaddr* to qemu_ram_ptr_length() but it wants a ram_addr_t*;
this will cause problems on 32 bit hosts and in any case
provokes a clang warning on MacOSX:

  CC    arm-softmmu/exec.o
exec.c:2164:46: warning: incompatible pointer types passing 'hwaddr *'
(aka 'unsigned long long *') to parameter of type 'ram_addr_t *'
(aka 'unsigned long *')
[-Wincompatible-pointer-types]
    return qemu_ram_ptr_length(raddr + base, plen);
                                             ^~~~
exec.c:1392:63: note: passing argument to parameter 'size' here
static void *qemu_ram_ptr_length(ram_addr_t addr, ram_addr_t *size)
                                                              ^

Since this function is only used in one place, change its
prototype to pass a hwaddr* rather than a ram_addr_t*,
rather than contorting the calling code to get the type right.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Tested-by: Riku Voipio <riku.voipio@linaro.org>
Tested-by: Peter Crosthwaite <peter.crosthwaite@xilinx.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

exec.c

@@ -1379,7 +1379,7 @@ static void *qemu_safe_ram_ptr(ram_addr_t addr)
 
 /* Return a host pointer to guest's ram. Similar to qemu_get_ram_ptr
  * but takes a size argument */
-static void *qemu_ram_ptr_length(ram_addr_t addr, ram_addr_t *size)
+static void *qemu_ram_ptr_length(ram_addr_t addr, hwaddr *size)
 {
     if (*size == 0) {
         return NULL;