tools: adjust --datadir option passed to qemu-upstream's configure script

Passing $(SHAREDIR)/qemu-xen isn't compatible with the way
os-posix.c:os_find_datadir() works. While one would generally expect
that function to honour the --datadir configure option, fixing this is
quite a bit more involved than simply adjusting the configure options
to match the function's behavior (in that, for an installed binary, it
looks in $bindir/../share/qemu). Afaict, so far this can have worked
only by virtue of what CONFIG_QEMU_DATADIR points to being populated
due to (presumably) some other qemu instance being installed on
people's systems (but being absent when running qemu-system-i386 from
the dist/ portion of the build tree).

Signed-off-by: Jan Beulich <JBeulich@xxxxxxxx>

--- a/tools/Makefile
+++ b/tools/Makefile
@@ -155,7 +155,7 @@ subdir-all-qemu-xen-dir subdir-install-q
 		--extra-ldflags="-L$(XEN_ROOT)/tools/libxc \
 		-L$(XEN_ROOT)/tools/xenstore" \
 		--bindir=$(LIBEXEC) \
-		--datadir=$(SHAREDIR)/qemu-xen \
+		--datadir=$(dir $(LIBEXEC))share/qemu \
 		--disable-kvm \
 		--python=$(PYTHON) \
 		$(IOEMU_CONFIGURE_CROSS); \