rtl8139: save/load RxMulOk counter (again)

Commit 9d29cdeaaca3a0383af764000b71492c4fc67c6e (rtl8139: port
TallyCounters to vmstate) introduced in incompatibility in the v4
format as it omitted the RxOkMul counter.

There are presumably no users that were impacted by the v4 to v4'
breakage, so increase the save version to 5 and re-add the field,
keeping backward compatibility with v4'.

We can't have a field conditional on the section version in
vmstate_tally_counters since this version checked would not be the
section version (but the version defined in this structure).  So, move
all the fields into the main state structure.

Signed-off-by: David Vrabel <david.vrabel@citrix.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>
