qtest: replace strtoXX() by qemu_strtoXX()

Check the result of qemu_strtoXX() and assert
if the string cannot be converted.

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