tests: don't check if qtest_spapr_boot() returns NULL

qtest_spapr_boot()/qtest_pc_boot()/qtest_boot() call qtest_vboot()
and qtest_vboot() calls g_malloc(),
and g_malloc() never fails:
if memory allocation fails, the application is terminated.

Signed-off-by: Laurent Vivier <lvivier@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Greg Kurz <groug@kaod.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
