libxl: CODING_STYLE: Much new material

Discuss:

    Memory allocation
    Conventional variable names
    Convenience macros
    Error handling
    Idempotent data structure construction/destruction
    Asynchronous/long-running operations

Signed-off-by: Ian Jackson <Ian.Jackson@eu.citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>
Release-Acked-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
libxl: CODING_STYLE: Deprecate `error' for out blocks

We should have only one name for this and `out' is more prevalent.

Signed-off-by: Ian Jackson <Ian.Jackson@eu.citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>
Release-Acked-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
libxl: CODING_STYLE: Mention function out parameters

We seem to use both `_r' and `_out'.  Document both.

Signed-off-by: Ian Jackson <Ian.Jackson@eu.citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>
Release-Acked-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
libxl: CODING_STYLE: Discuss existing style problems

Document that:
 - the existing code is not all confirming yet
 - code should conform
 - we will sometimes accept patches with nonconforming elements if
   they don't make matters worse.

Signed-off-by: Ian Jackson <Ian.Jackson@eu.citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>
Release-Acked-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>