tools/libxl: Work around libvirt breakage in libxl__cpuid_legacy()

tools/libxl: Work around libvirt breakage in libxl__cpuid_legacy()

OSSTest reports that libvirt is reliably regressed.

The only possible option is a side effect of using libxl_defbool_val(), which
can only be the assert() within.  Unfortunately, libvirt actually crashes in
__vfscanf_internal() while presumably trying to render some form of error.

Open code the check without the assert() to unblock staging, while we
investigate what is going on with libvirt.  This will want reverting at some
point in the future.

Not-really-fixes: bfcc97c08c ("tools/cpuid: Plumb nested_virt down into xc_cpuid_apply_policy()" reliably breaks libvirt.)
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Ian Jackson <iwj@xenproject.org>