checkpatch: bump most warnings to errors

This only leaves a warning-level message for the extra-long lines
soft limit.  Everything else is bumped up.

In the future warnings can be added for checks that can have false
positives.

Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
