tests.pxe: Add a subtest of pxe

Signed-off-by: NJason Wang <jasowang@redhat.com>
Signed-off-by: NAmos Kong <akong@redhat.com>

tests/cfg/pxe.cfg

@@ -15,6 +15,11 @@
     image_verify_bootable = no
     variants:
         - @default:
+        - etherboot:
+            pre_command = "alternatives --display qemu-pxe-roms |grep -q etherboot && alternatives --set 'qemu-pxe-roms' /usr/share/etherboot || true"
+            no Host_RHEL.6
+        - gpxe:
+            pre_command = "alternatives --display qemu-pxe-roms |grep -q gpxe || alternatives --install /usr/share/qemu-pxe-roms qemu-pxe-roms /usr/share/gpxe 1; alternatives --set 'qemu-pxe-roms' /usr/share/gpxe"
         - with_query_cpus:
             type = pxe_query_cpus
             start_vm = no