qapi: dealloc visitor, support freeing of nested lists

qapi: dealloc visitor, support freeing of nested lists

Previously our logic for keeping track of when we're visiting the head
of a list was done via a global bool. This can be overwritten if dealing
with nested lists, so use stack entries to track this instead.

Reviewed-by: Anthony Liguori <aliguori@us.ibm.com>
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>