Forbidden pxe_boot test for igb nic

Based on the developer comment forbbiden PXE tests, since igb is not
supported yet in BIOS.

Signed-off-by: Lei Yang <leiayang@redhat.com>

generic/tests/cfg/pxe_boot.cfg

@@ -24,6 +24,7 @@
     # capture the packet from the macvtap tap port, disable it
     # for macvtap temporarily, will fix it in the furture.
     no macvtap
+    no igb
     variants:
         - @default:
         - gpxe: