spapr_vio: Pair g_malloc() with g_free(), not free()

Spotted by Coverity with preview checker ALLOC_FREE_MISMATCH enabled
and my "coverity: Model g_free() isn't necessarily free()" model patch
applied.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Gonglei <arei.gonglei@huawei.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
