xen/arm: traps: Don't ignore invalid value for serrors=

xen/arm: traps: Don't ignore invalid value for serrors=

serrors= only supports 3 values "diverse", "forward" and "panic".

The current implementation of parse_serrors_behavior() will default to
"diverse" for any invalid value and not tell the users.

Rather than ignore the invalid input, return an error to the caller so
it can decides the be approach.

This will be useful after a follow-up patch where the number of options
will be reduced.

Take the opportunity to initialize serrors_op to SERRORS_DIVERSE rather
than relying on the item to be the first in the enum and therefore
equal to 0.

Signed-off-by: Julien Grall <julien.grall@arm.com>
Reviewed-by: Stefano Stabellin <sstabellini@kernel.org>
Release-acked-by: Juergen Gross <jgross@suse.com>