libxl: Compile with -Wformat-nonliteral.

libxl: Compile with -Wformat-nonliteral.

At least one compiler (some Ubuntu version) uses this by default and it seems
like a good idea anyway and the fixup required is trivial.

One hunk is from a patch by Ian Jackson.

Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>
Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Reported-by: Adda Rathbone <addarathbone@googlemail.com>
Tested-by: Adda Rathbone <addarathbone@googlemail.com>
Tested-by: Andrew Pounce <andrew.pounce@citrix.com>
Committed-by: Ian Jackson <ian.jackson@eu.citrix.com>