docs: add a document describing the xl cfg file syntax

docs: add a document describing the xl cfg file syntax

Based on an initial version by Ian Jackson.

I believe that all keys are now present in the document although there are are
various omissions in the actual documentation of them. Hopefully however this
covers the majority of the most interesting keys.

Spice section:
Signed-off-by: Zhou Peng <zhoupeng@nfs.iscas.ac.cn>
Committed-by: Ian Jackson <ian.jackson.citrix.com>
Acked-by: Ian Jackson <ian.jackson.citrix.com>
The rest:
Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Committed-by: Ian Jackson <ian.jackson.citrix.com>
Acked-by: Ian Jackson <ian.jackson.citrix.com>