api: disallow virDomainSaveImageGetXMLDesc on read-only connections

api: disallow virDomainSaveImageGetXMLDesc on read-only connections

The virDomainSaveImageGetXMLDesc API is taking a path parameter,
which can point to any path on the system. This file will then be
read and parsed by libvirtd running with root privileges.

Forbid it on read-only connections.

Fixes: CVE-2019-10161
Reported-by: Matthias Gerstner <mgerstner@suse.de>
Signed-off-by: Ján Tomko <jtomko@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>