tap-win32: skip unexpected nodes during registry enumeration

In order to find a named tap device, get_device_guid() enumerates children of
HKLM\SYSTEM\CCS\Control\Network\{4D36E972-E325-11CE-BFC1-08002BE10318}
(aka NETWORK_CONNECTIONS_KEY). For each child, it then looks for a
"Connection" subkey, but if this key doesn't exist, it aborts the
entire search. This was observed to fail on at least one Windows 10
machine, where there is an additional child of NETWORK_CONNECTIONS_KEY
(named "Descriptions"). Since registry enumeration doesn't guarantee
any particular sort order, we should continue to search for matching
children rather than aborting the search.

Signed-off-by: Andrew Baumann <Andrew.Baumann@microsoft.com>
Reviewed-by: Stefan Weil <sw@weilnetz.de>
Signed-off-by: Jason Wang <jasowang@redhat.com>