Search $bindir/../share/qemu for datadir

This restores the behavior that was removed by 6dd2dace.
diff --git a/os-posix.c b/os-posix.c
index 3cd52e1..3befc0e 100644
--- a/os-posix.c
+++ b/os-posix.c
@@ -89,6 +89,7 @@
 {
     g_autofree char *exec_dir = NULL;
     g_autofree char *dir = NULL;
+    g_autofree char *share_dir = NULL;
 
     exec_dir = qemu_get_exec_dir();
     g_return_val_if_fail(exec_dir != NULL, NULL);
@@ -98,6 +99,11 @@
         return g_steal_pointer(&dir);
     }
 
+    share_dir = g_build_filename(exec_dir, "..", "share", "qemu", NULL);
+    if (g_file_test(share_dir, G_FILE_TEST_IS_DIR)) {
+        return g_steal_pointer(&share_dir);
+    }
+
     return g_strdup(CONFIG_QEMU_DATADIR);
 }