virsysinfo: Calculate OEM string index better

virsysinfo: Calculate OEM string index better

As can be seen in earlier commits, there can be two OEM strings
with the same index. But since our parser
(virSysinfoParseOEMStrings()) doesn't expect that, it increments
index in each run and thus skips over these strings.
Fortunately, we have the right index at hand - we're just
skipping over it in a loop. Just reconstruct the index back
inside the loop.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Jiri Denemark <jdenemar@redhat.com>