Revert "qtest: Fix crash if SIGABRT during qtest_init()"

It turns out there are test cases that use multiple libqtest instances.
We cannot use a global qtest instance in the SIGABRT handler.

This reverts commit cb201b4872f16dfbce63f8648b2584631e2e965f.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Marcel Apfelbaum <marcel.a@redhat.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>
