live-vm-common.bbclass: Allow to use different kernel image than KERNEL_IMAGETYPE for /vmlinuz
* syslinux config hardcodes kernel image as /vmlinuz add warning message
when the selected image doesn't exist and allow to select different image
with VM_DEFAULT_KERNEL variable (qemuboot.bbclass is using QB_DEFAULT_KERNEL)
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com> Signed-off-by: Ross Burton <ross.burton@intel.com>