libxl: eliminate memory leak in libxmlDomainModifyDeviceFlags

This call to virDomainDeviceDefParse is both unnecessary (since
it will again be called at the top of the immediately following if(),
and if not there, then at the top of the if following that), but it
also creates a leak of one virDomainDeviceDef and one [whatever type
of device the DeviceDef is pointing to; probably a virDomainDiskDef]
in the case that the function has been called with
VIR_DOMAIN_DEVICE_MODIFY_CONFIG (the second parse will overwrite the
devicedef that was just created).

src/libxl/libxl_driver.c

@@ -3243,10 +3243,6 @@ libxlDomainModifyDeviceFlags(virDomainPtr dom, const char *xml,
          goto cleanup;
     }
 
-    if (!(dev = virDomainDeviceDefParse(driver->caps, vm->def, xml,
-                                  VIR_DOMAIN_XML_INACTIVE)))
-        goto cleanup;
-
     priv = vm->privateData;
 
     if (flags & VIR_DOMAIN_DEVICE_MODIFY_CONFIG) {