machine defaults to find_default_machine(),
then gets overridden via -M and machine_parse().

If no -M is specified and find_default_machine() returns NULL
(when no machine compiled in), exit with an error.

Avoids a segfault when setting machine->max_cpus.
Signed-off-by: NAndreas Färber <andreas.faerber@web.de>
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>