x86emul: drop pointless and add useful default cases

x86emul: drop pointless and add useful default cases

There's no point in having default cases when all possible values have
respective case statements, or when there's just a "break" statement.

Otoh the two main switch() statements better get default cases added,
just to cover the case of someone altering one of the two lookup arrays
without suitably changing these switch statements.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>