api: disallow virConnectGetDomainCapabilities on read-only connections

This API can be used to execute arbitrary emulators.
Forbid it on read-only connections.

Fixes: CVE-2019-10167
Signed-off-by: Ján Tomko <jtomko@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
(cherry picked from commit 8afa68bac0)
Signed-off-by: Ján Tomko <jtomko@redhat.com>

src/libvirt-domain.c

@@ -11117,6 +11117,7 @@ virConnectGetDomainCapabilities(virConnectPtr conn,
     virResetLastError();
 
     virCheckConnectReturn(conn, NULL);
+    virCheckReadOnlyGoto(conn->flags, error);
 
     if (conn->driver->connectGetDomainCapabilities) {
         char *ret;