lscpu: theoretical buffer overflow

there is a theoretical buffer overflow possible in the hypervisor
parsing code of lscpu.  It would require a proc entry to return way more
than expected so it's no high priority.  But better be safe than sorry.

At first I thought about switching to fgets but there is another
code file that adds a format specifier.  The diff is less intrusive
that way, too.

Signed-off-by: Karel Zak <kzak@redhat.com>