Commit db64d32 breaks CentOS 6.6 unattended install

[hpaluch-pil]

The commit db64d32, especially this part:

db64d32#diff-0e7a91288892b5925c13e11a6047eb91fb7dd34309add1f137c0d9c0b9e3e536R8

diff --git a/shared/cfg/guest-os/Linux/CentOS/6.6.cfg b/shared/cfg/guest-os/Linux/CentOS/6.6.cfg
index ee560db8..930da997 100644
--- a/shared/cfg/guest-os/Linux/CentOS/6.6.cfg
+++ b/shared/cfg/guest-os/Linux/CentOS/6.6.cfg
@@ -5,6 +5,7 @@
     unattended_install, check_block_size.4096_512, check_block_size.512_512, svirt_install:
         unattended_file = unattended/CentOS-6-6.ks
         syslog_server_proto = udp
+        kernel_params = ""
     nic_hotplug:
         modprobe_module =
     block_hotplug:

Breaks unattended install of CentOS 6.6, because it completely erases boot parameters necessary for serial console setup and ks=cdrom.

Proposed fix is to remove line kernel_params = "" entirely as it was done for other distributions in ths commit: db64d32

Example commands to reproduce this bug (tested on Fedora 35 with Avocado 90 official RPMs):

avocado vt-bootstrap --vt-no-downloads
mkdir ~/avocado/data/avocado-vt/isos/linux
cd ~/avocado/data/avocado-vt/isos/linux
curl -fLO https://vault.centos.org/6.6/isos/x86_64/CentOS-6.6-x86_64-minimal.iso
avocado run \
   --vt-guest-os Guest.Linux.CentOS.6.6.x86_64 \
   unattended_install.cdrom.extra_cdrom_ks.default_install.aio_threads