oslib: do not call g_strdup from qemu_get_exec_dir

Just return the directory without requiring the caller to free it.
This also removes a bogus check for NULL in os_find_datadir and
module_load_one; g_strdup of a static variable cannot return NULL.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

os-win32.c

@@ -65,7 +65,7 @@ void os_setup_early_signal_handling(void)
  */
 char *os_find_datadir(void)
 {
-    return qemu_get_exec_dir();
+    return g_strdup(qemu_get_exec_dir());
 }
 
 void os_set_line_buffering(void)