conf: eliminate redundant use of VIR_ALLOC

conf: eliminate redundant use of VIR_ALLOC

We can use VIR_REALLOC_N with NULL pointer, which behaves the same way
as VIR_ALLOC_N in that case, so no need for a condition that's
checking if some data are allocated already.

---

I tried to find other parts of the code similar to this, so I can do a
full cleanup for the whole repository, so I used this (excuse the long
line, but that's how I was writing it):

git grep -nHC 5 -e VIR_REALLOC_N -e VIR_ALLOC_N | while read line; do if [[ "$line" == "--" ]]; then if [[ ${#tmpbuf} -gt 10 && "$REALLOC_N" == "true" && "$ALLOC_N" == "true" ]]; then echo $line; while [[ ${#tmpbuf[*]} -gt 0 ]]; do echo "${tmpbuf[0]}"; tmpbuf=( "${tmpbuf[@]:1:${#tmpbuf[*]}}" ); done; fi; unset tmpbuf REALLOC_N ALLOC_N; else if [[ "$ALLOC_N" != "true" && "${line/VIR_ALLOC_N//}" != "${line}" ]]; then ALLOC_N="true"; fi; if [[ "$REALLOC_N" != "true" && "${line/VIR_REALLOC_N//}" != "${line}" ]]; then REALLOC_N="true"; fi; tmpbuf[${#tmpbuf[*]}]="$line"; fi; done | less

And reviewed the output just to find out this was the only occurrence of
the inconsistency.