machine: Improve is_cpu_type_supported()

machine: Improve is_cpu_type_supported()

It's no sense to check the CPU type when mc->valid_cpu_types[0] is
NULL, which is a program error. Raise an assert on this.

A precise hint for the error message is given when mc->valid_cpu_types[0]
is the only valid entry. Besides, enumeration on mc->valid_cpu_types[0]
when we have mutiple valid entries there is avoided to increase the code
readability, as suggested by Philippe Mathieu-Daudé.

Besides, @cc comes from machine->cpu_type or mc->default_cpu_type. For
the later case, it can be NULL and it's also a program error. We should
use assert() in this case.

Signed-off-by: Gavin Shan <gshan@redhat.com>
Message-ID: <20231204004726.483558-4-gshan@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>