[xmlsec] Add key to keysmanager

[Aleksey Sanin]

> Fuction xmlSecMSCryptoAppDefaultKeysMngrAdoptKey or
> xmlSecSimpleKeysStoreAdoptKey always return -1. What do I wrong?

You should have an error reported. If you are writing a GUI
application w/o console then you probably do not see it. Thus,
you might want to put a breakpoint here to get it manually:

http://www.aleksey.com/xmlsec/api/xmlsec-errors.html#XMLSECERRORSDEFAULTCALLBACK

Aleksey