kvm: for disabling pxe, pass empty file

Passing the file argument to the xml break for EL 7.1, the fix removes
the argument as just passing rombar='off' with its file arg to be empty string.

Signed-off-by: Rohit Yadav <rohit.yadav@shapeblue.com>

Author: yadvr

plugins/hypervisors/kvm/src/com/cloud/hypervisor/kvm/resource/LibvirtVMDef.java

@@ -967,7 +967,7 @@ public String toString() {
                 netBuilder.append("<script path='" + _scriptPath + "'/>\n");
             }
             if (_pxeDisable) {
-                netBuilder.append("<rom bar='off' file='dummy'/>");
+                netBuilder.append("<rom bar='off' file=''/>");
             }
             if (_virtualPortType != null) {
                 netBuilder.append("<virtualport type='" + _virtualPortType + "'>\n");