kvm: use linux kernel image from /boot/ folder for kvm test

Linux kernel image for refinfra platforms is located in
/boot/vmlinux-refinfra. Use this image for the kvm test.

Change-Id: Iabefa95fefc0f7c64ef41b7a800249958d100242
Signed-off-by: Vijayenthiran Subramaniam <vijayenthiran.subramaniam@arm.com>