tests: reintroduce tests for libxl's legacy nested setting

The preferred location for setting the nested CPU flag changed in
Xen 4.10 and is advertised via the LIBXL_HAVE_BUILDINFO_NESTED_HVM
define. Commit 95d19cd0 changed libxl to use the new preferred
location but unconditionally changed the tests, causing 'make check'
failures against Xen < 4.10 that do not contain the new location.

Commit e94415d5 fixed the failures by only running the tests when
LIBXL_HAVE_BUILDINFO_NESTED_HVM is defined. Since libvirt supports
several versions of Xen that use the old nested location, it is
prudent to test the flag is set correctly. This patch reintroduces
the tests for the legacy location of the nested setting.

Signed-off-by: Jim Fehlig <jfehlig@suse.com>
Reviewed-by: Erik Skultety <eskultet@redhat.com>