qemu: make separate function for setting statsType of privateData

qemu: make separate function for setting statsType of privateData

We only need to set statsType in almost every case of setting
something from private data, so it seems unnecessary to pull
privateData out of current / completed job for just this one
thing every time. I think this patch keeps the code cleaner
without variables used just once.

Signed-off-by: Kristina Hanicova <khanicov@redhat.com>
Reviewed-by: Jiri Denemark <jdenemar@redhat.com>