hostmem: Validate host-nodes before setting bitmap

hostmem: Validate host-nodes before setting bitmap

host_memory_backend_set_host_nodes() was not validating
host-nodes before writing to backend->host_nodes, making QEMU
write beyond the end of the bitmap.

Fix the crash and add a simple regression test for the fix.

While at it, fix memory leak of the list returned by
visit_type_uint16List().

Reported-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Message-Id: <20181130122844.29103-1-ehabkost@redhat.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Igor Mammedov <imammedo@redhat.com>
[ehabkost: removed test case code]
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>