xc_cpuid_set() returns allocated memory via cpuid_res, which libxl needs to
free() seeing as it discards the results.
This is logically a backport of c/s b91825f628 "tools/libxc: Drop
config_transformed parameter from xc_cpuid_set()" but rewritten as one caller
of xc_cpuid_set() does use returned values.
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>