qemuDomainBlocksStatsGather: Fix blockstats gathering after refactor

qemuDomainBlocksStatsGather: Fix blockstats gathering after refactor

Commit 58aa005f3e95114 which refactored how block stats are stored
intended to change the code path where stats for all devices are totaled
together by allocating new stats object and using that but the commit
forgot to actually change the pointers inside the loop.

Unfortunately this was not caught by the compiler as there were
pre-existing pointers of the same type with the same name, which
resulted into a NULL dereference.

Fixes: 58aa005f3e95114b4f2dab76ee4ade06182a3f20
Closes: https://gitlab.com/libvirt/libvirt/-/issues/827
Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Jiri Denemark <jdenemar@redhat.com>