A comment by Jarrod
sudo sed -i -E '/^\s+linux/ s/$/ kvm-intel.vmentry_l1d_flush=always/' grub.cfg
Above sed command will append 'kvm-intel.vmentry_l1d_flush=always' to lines in 'grub.cfg' starting with word 'linux' (prefixed with whitespace) to file 'grub.cfg'