util: make forgotten changes suggested during review of commit d40b820c

util: make forgotten changes suggested during review of commit d40b820c

I had intended to make these changes to commit d40b820c before
pushing, but forgot about it during the day between the initial review
and ACK.

Neither change is significant - just returning immediately when
virNetDevGetName() fails (instead of logging a debug message first)
and eliminating a comment that adds to confusion rather than
eliminating it. Still, the changes should be made to be more
consistent with nearly identical code just a few lines up (added in
commit 7282f455)

Signed-off-by: Laine Stump <laine@laine.org>
Reviewed-by: Ján Tomko <jtomko@redhat.com>