qtest: Fix crash if SIGABRT during qtest_init()

If an assertion fails during qtest_init() the SIGABRT handler is
invoked.  This is the correct behavior since we need to kill the QEMU
process to avoid leaking it when the test dies.

The global_qtest pointer used by the SIGABRT handler is currently only
assigned after qtest_init() returns.  This results in a segfault if an
assertion failure occurs during qtest_init().

Move global_qtest assignment inside qtest_init().  Not pretty but let's
face it - the signal handler depends on global state.

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