libxl/CODING_STYLE: Clarify the singular statement in a conditional statement explana...

libxl/CODING_STYLE: Clarify the singular statement in a conditional statement explanation.

It takes a bit of thinking to parse the original statement. It looks
like it is missing an ',' - right after the 'braced' to make it
obvious that:

if (X)
   return XYZ;

is OK, while multiple conditionals require braces.

Changing the 'apart from' with 'except' makes it more obvious
(I hope).

Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>