tools/libxc: Fix error handling in get_cpuid_domain_info()

tools/libxc: Fix error handling in get_cpuid_domain_info()

get_cpuid_domain_info() has two conflicting return styles - either -error for
local failures, or -1/errno for hypercall failures.  Switch to consistently
use -error.

While fixing the xc_get_cpu_featureset(), take the opportunity to remove the
redundancy and move it to be adjacent to the other featureset handling.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>