test-vmstate: fix bad GTree usage, use-after-free

test-vmstate: fix bad GTree usage, use-after-free

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.

Fixes: 9a85e4b8f6 ("migration: Support gtree migration")
Resolves: https://gitlab.com/qemu-project/qemu/-/issues/1518
Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Eric Auger <eric.auger@redhat.com>
Reported-by: Richard W.M. Jones <rjones@redhat.com>
Tested-by: Richard W.M. Jones <rjones@redhat.com>
Reviewed-by: Richard W.M. Jones <rjones@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>