[qe-core] Reset ppc nvram before trying to boot Linux kernel to prevent PXE related file reading timeouts
as observed in #126821 sporadic failures occur that grub related files or linux kernel or initrd can not be read properly during a PXE boot. Most prominent error "error: timeout reading" from a grub prompt as visible in https://openqa.suse.de/tests/11162714#step/bootloader/26 . The fix for that seems to be to clean the LPARs NVRAM so openQA test code instrumenting the PPC SMS menu should include that step automatically to ensure proper PXE booting.
- AC1: openQA tests on ppc64le hvm (and potentially spvm) backend trigger an NVRAM cleanup before trying PXE boot
- Follow the section "Resolving The Problem" from https://www.ibm.com/support/pages/could-not-allocate-memory-rtas-grub-or-yaboot-ppcbe and automate that as part of test automation in os-autoinst-distri-opensuse
- Apply the NVRAM cleanup manually for affected machines