conf: sanitize virDomainSaveStatus & virDomainSaveConfig APIs

conf: sanitize virDomainSaveStatus & virDomainSaveConfig APIs

Our normal practice is for the object type to be the name prefix, and
the object instance be the first parameter passed in.

Rename these to virDomainObjSave and virDomainDefSave moving their
primary parameter to be the first one. Ensure that the xml options
are passed into both functions in prep for future work.

Finally enforce checking of the return type and mark all parameters
as non-NULL.

Reviewed-by: Michal Privoznik <mprivozn@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>