tools/libxl: Re-position CPUID handling during domain construction

tools/libxl: Re-position CPUID handling during domain construction

CPUID handling needs to be earlier in construction.  Move it from its current
position in libxl__build_post() to libxl__build_pre() for fresh builds, and
libxl__srm_callout_callback_static_data_done() for the migration/resume case.

Later changes will make the migration/resume case conditional on whether CPUID
data was present in the migration stream, and the libxc layer took care of
restoring it.

No functional change.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>