tests/qtest: fix registration of ABRT handler for QEMU cleanup

qtest_init registers a hook to cleanup the running QEMU process
should g_assert() fire before qtest_quit is called. When the first
hook is registered, it is supposed to triggere registration of the
SIGABRT handler. Unfortunately the logic in hook_list_is_empty is
inverted, so the SIGABRT handler never gets registered, unless
2 or more QEMU processes are run concurrently. This caused qtest
to leak QEMU processes anytime g_assert triggers.

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
Message-Id: <20220513154906.206715-2-berrange@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>