api: disallow virDomainManagedSaveDefineXML on read-only connections

api: disallow virDomainManagedSaveDefineXML on read-only connections

The virDomainManagedSaveDefineXML can be used to alter the domain's
config used for managedsave or even execute arbitrary emulator binaries.
Forbid it on read-only connections.

Fixes: CVE-2019-10166
Reported-by: Matthias Gerstner <mgerstner@suse.de>
Signed-off-by: Ján Tomko <jtomko@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
(cherry picked from commit db0b78457f183e4c7ac45bc94de86044a1e2056a)
Signed-off-by: Ján Tomko <jtomko@redhat.com>