spapr: Fix compiler warnings for some versions of gcc

spapr: Fix compiler warnings for some versions of gcc

i686-w64-mingw32-gcc (GCC) 4.6.3 from Debian wheezy reports these warnings:

hw/ppc/spapr_hcall.c:188:1: warning:
 control reaches end of non-void function [-Wreturn-type]

hw/ppc/spapr_pci.c:454:1: warning:
 control reaches end of non-void function [-Wreturn-type]

Both warnings are fixed by using g_assert_not_reached instead of assert.
A second line with assert(0) in spapr_pci.c which did not raise a compiler
warning was modified, too, because g_assert_not_reached documents the
purpose of that statement and is not removed in release builds.

Signed-off-by: Stefan Weil <sw@weilnetz.de>
Acked-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Alexander Graf <agraf@suse.de>