xend: Only add cpuid and cpuid_check to sexpr once

xend: Only add cpuid and cpuid_check to sexpr once

tools/xend: Only add cpuid and cpuid_check to sexpr once

When converting a XendConfig object to sexpr, cpuid and cpuid_check
were being emitted twice in the resulting sexpr.  The first conversion
writes incorrect sexpr, causing parsing of the sexpr to fail when xend
is restarted and domain sexpr files in /var/lib/xend/domains/<dom-uuid>
are read and parsed.

This patch skips the first conversion, and uses only the custom
cpuid{_check} conversion methods called later.  It is not pretty, but
is the least invasive fix in this complex code.

Signed-off-by: Olaf Hering <olaf@aepfle.de>