According to g_tree_foreach() documentation:
"The tree may not be modified while iterating over it (you can't
add/remove items)."
compare_trees()/diff_tree() fail to respect this rule.
Historically GLib2 used a slice allocator for the GTree APIs
which did not immediately release the memory back to the system
allocator. As a result QEMU's use-after-free bug was not visible.
With GLib > 2.75.3 however, GLib2 has switched to using malloc
and now a SIGSEGV can be observed while running test-vmstate.
Get rid of the node removal within the tree traversal. Also
check the trees have the same number of nodes before the actual
diff.