core: Allow enabling/disabling SSO

This small feature allows controlling SSO per VM.
Changes:
 - new attribute of VM/Template/Pool SsoMethod
 - SsoMethod is an enum that describes which sso method will be used
   (for now only two options are possible [disable sso/use guest
   agent]). In future this can be extended. Default sso method is to use
   guest agent.

Change-Id: I175ecd61c35b8354706d6dedda1e211bf2400c54
Signed-off-by: Frantisek Kobzik <fkobzik@redhat.com>
Bug-Url: https://bugzilla.redhat.com/1035279