qemu: Bring if() outside from loop in virDomainFSInfoFormat()

qemu: Bring if() outside from loop in virDomainFSInfoFormat()

After previous commit, the freeing of @info_ret inside of
virDomainFSInfoFormat() looks like this:

  for () {
    if (info_ret)
      virDomainFSInfoFree(info_ret[i]);
  }

It is needless to compare @info_ret against NULL in each
iteration. We can switch the order and do the comparison first
followed by the loop.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>