libxl: capsInitCPU: fail if we can't initialize host features

libxl: capsInitCPU: fail if we can't initialize host features

Introduced by:
  commit 17322e551861a5bc4dd464a7c5399204dc8c7caa
      libxl: describe host cpu features based on hwcaps
with the justification that libxl_hwcaps does not have a stable
format across all version.

Even though the code would return '0' in the case of such failure,
it frees the 'cpu' pointer, while keeping it in caps->host.
Based on that, assume it does not happen in current usage.

Signed-off-by: Ján Tomko <jtomko@redhat.com>
Reviewed-by: Jim Fehlig <jfehlig@suse.com>