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: NMatthias Gerstner <mgerstner@suse.de>
Signed-off-by: NJán Tomko <jtomko@redhat.com>
Reviewed-by: NDaniel P. Berrangé <berrange@redhat.com>

src/libvirt-domain.c

@@ -9570,6 +9570,7 @@ virDomainManagedSaveDefineXML(virDomainPtr domain, const char *dxml,
 
     virCheckDomainReturn(domain, -1);
     conn = domain->conn;
+    virCheckReadOnlyGoto(conn->flags, error);
 
     if (conn->driver->domainManagedSaveDefineXML) {
         int ret;