m4: enforce that all enum cases are listed in switch statements

m4: enforce that all enum cases are listed in switch statements

As a general rule any time we switch() on something that is an enum, we
want to have a case for every enum constant. The -Wswitch warning will
report any switch where we've violated this rule, except if that switch
has a default case.

Unfortunately it is reasonable to want to list all enum constants *and*
also have a default case. To get a warning in that scenario requires
that we turn on -Wswitch-enum.

In a few cases where we explicitly don't want to list all enum cases, we
can discard the enum type checking by casting the value to a plain int.

Reviewed-by: John Ferlan <jferlan@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>