gdm: Refactor oVirt to a generic CredentialManager interface

gdm: Refactor oVirt to a generic CredentialManager interface

    Commit 4cda61a1 added support for pre-authenticated logins in
    oVirt environments. This feature prevents a user from having
    to type their password twice (once to the oVirt management machine,
    and then immediately again in the provisioned guest running gnome-shell).
    That feature is currently oVirt specific, but a similar feature would
    be useful in non-oVirt based virt farm environments.

    Toward that end, this commit generalizes the various aspects of the
    oVirt integration code, so that it can be reused in a subsequent
    commit for adding single sign on support in vmware deployments, too.

    Closes: https://gitlab.gnome.org/GNOME/gnome-shell/issues/1983