xenapi: allow non rsa key injection

Currently if you attempt to inject a dsa key into a server, you are
unable to start the server because the saving of the password will
always fail. This ensures the saving of the password is only attempted
when injecting an rsa based key.

Fixes bug 1206458
Change-Id: I85e259c7d61182d32d2661662483237e7ad8feed
Merge "xenapi: allow non rsa key injection"