qdict: fix unbounded stack warning for qdict_array_entries

qdict: fix unbounded stack warning for qdict_array_entries

Here we use one g_strdup_printf() to replace the two stack allocated
array, considering it's more convenient, safe, and as long as it's
called rarely only when quorum device opens. This will remove the
unbound stack warning when compiling with "-Wstack-usage=1000000".

Reviewed-by:   Eric Blake <eblake@redhat.com>
Signed-off-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>