libxlutil: avoid almost-undefined behavior

libxlutil: avoid almost-undefined behavior

While only value computations of an object are disallowed in the
presence of another unsequenced side effect, at least gcc 4.3 looks to
extend this to taking the object's address. The resulting warning causes
the build to fail, because of -Werror.

While there also correct an adjacent comment.

Fixes: bdc0799fe26a ("libxlu: introduce xlu_pci_parse_spec_string()")
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Ian Jackson <ian.jackson@eu.citrix.com>