vz: declare missing driver variable

This fixes commit 61bff77bSigned-off-by: NDaniel P. Berrangé <berrange@redhat.com>

src/vz/vz_driver.c

@@ -718,6 +718,7 @@ static char *
 vzDomainGetXMLDesc(virDomainPtr domain, unsigned int flags)
 {
     vzConnPtr privconn = domain->conn->privateData;
+    vzDriverPtr driver = privconn->driver;
     virDomainDefPtr def;
     virDomainObjPtr dom;
     char *ret = NULL;